Circular 1360

Water Quality in Principal Aquifers

of the United States, 1991–2010

National Water-Quality Assessment Program

U.S. Department of the Interior 

U.S. Geological Survey

The Quality of Our Nation’s Waters
Front cover. Map showing Principal Aquifers of the United States that are included in this study. Image of pitcher and glass copyright 

istockphoto.com.

Back cover. USGS scientists collecting and processing samples from monitoring, domestic, and public-supply wells for this study. 

Photographs by USGS.
Water Quality in Principal Aquifers 

of the United States, 1991–2010

By Leslie A. DeSimone, Peter B. McMahon, and Michael R. Rosen

National Water-Quality Assessment Program

Circular 1360

U.S. Department of the Interior

U.S. Geological Survey

The Quality of Our Nation’s Waters
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

DeSimone, L.A., McMahon, P.B., and Rosen, M.R., 2014, The quality of our Nation’s waters—Water quality in Principal 

Aquifers of the United States, 1991–2010: U.S. Geological Survey Circular 1360, 151 p., http://dx.doi.org/10.3133/cir1360.

Library of Congress Cataloging-in-Publication Data

Desimone, Leslie A.

 The quality of our nation’s waters- - -water quality in principal aquifers of the United States, 

1991--2010 / by Leslie A. DeSimone, Peter B. McMahon, and Michael R. Rosen.

 pages cm. -- (Circular ; 1360)

 Other title: Water quality in principal aquifers of the United States, 1991--2010

 ISBN 978-1-4113-3758-9 (alk. paper)

 1. Groundwater--Quality--United States. 2. Aquifers--United States. I. McMahon, Peter B. II. 

Rosen, Michael R., 1961- III. Geological Survey (U.S.) IV. Title. V. Title: Water quality in principal aquifers of the United 

States, 1991--2010. VI. Series: U.S. Geological Survey circular ; 1360.

 GB1015.D47 2014

 553.7’9097309049--dc23

 2014031450

ISSN 1067–084X (print)

ISSN 2330–5703 (online)

ISBN 978-1-4113-3758-9 
iii

Foreword

The United States has made major investments in assessing, managing, regulating, and 

conserving natural resources, such as water and a variety of ecosystems. Sustaining the quality 

of the Nation’s water resources and the health of our diverse ecosystems depends on the 

availability of sound water-resources data and information to develop effective, science-based 

policies. Effective management of water resources also brings more certainty and efficiency 

to important economic sectors. Taken together, these actions lead to immediate and long-term 

economic, social, and environmental benefits that make a difference to the lives of millions of 

people (http://water.usgs.gov/nawqa/applications/).

Two decades ago, Congress established the U.S. Geological Survey’s National Water-Quality 

Assessment (NAWQA) Program to meet this need. Since then NAWQA has served as a primary 

source of nationally consistent information on the quality of the Nation’s streams and groundwater, on ways in which water quality changes over time, and on the natural features and 

human activities affecting the quality of streams and groundwater. Objective and reliable data, 

systematic scientific studies, and models are used to characterize where, when, and why the 

Nation’s water quality is degraded—and what can be done to improve and protect the water for 

human and ecosystem needs. This information is critical to our future because the Nation faces 

an increasingly complex and growing need for clean water to support people, economic growth, 

and healthy ecosystems. For example, NAWQA findings for public-supply wells, which provide 

water to about 105 million people, showed that 22 percent of source-water samples contained 

at least one contaminant at levels of potential health concern. Similarly, 23 percent of samples 

from domestic (or privately owned) wells, which supply untreated water to an additional 43 million people, also had contaminant levels of potential concern. 

This report is one of a collection of publications that describe water-quality conditions in 

selected Principal Aquifers of the United States (http://water.usgs.gov/nawqa/studies/praq/). 

The collection is part of the series “The Quality of Our Nation’s Waters,” which describes major 

findings of the NAWQA Program on water-quality issues of regional and national concern 

and which provides science-based information for assessing and managing the quality of our 

groundwater resources. Other reports in this series focus on occurrence and distribution of 

nutrients, pesticides, and volatile organic compounds in streams and groundwater, the effects 

of contaminants and streamflow alteration on the condition of aquatic communities in streams, 

and the quality of untreated water from private domestic and public-supply wells. Each report 

builds toward a more comprehensive understanding of the quality of regional and national 

water resources (http://water.usgs.gov/nawqa/nawqa_sumr.html ). All NAWQA reports are 

available online at http://water.usgs.gov/nawqa/bib/.

The information in this series primarily is intended for those interested or involved in resource 

management and protection, conservation, regulation, and policymaking at regional and 

national levels. In addition, the information should be of interest to those at a local level 

who wish to know more about the general quality of streams and groundwater in areas near 

where they live and how that quality compares with other areas across the Nation. We hope 

this publication will provide you with insights and information to meet your needs and will 

foster increased citizen awareness and involvement in the protection and restoration of our 

Nation’s waters.

Jerad Bales

Acting Associate Director for Water

U.S. Geological Survey 
National Water-Quality Assessment Program
v

Contents

Chapter 1: Overview of Major Findings and Implications....................................................................1

Chapter 2: Principal Aquifers and NAWQA Approach to Assessing Groundwater Quality...........9

Chapter 3: Aquifers and Processes That Affect Groundwater Quality ...........................................19

Chapter 4: The Quality of Groundwater Resources For Drinking and Other Uses ........................35

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur...........47

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur..........81

Chapter 7: How Does Our Use of Water Affect Groundwater Quality? .........................................109

Chapter 8: How Is Groundwater Quality Changing? .........................................................................125

References Cited.......................................................................................................................................134

Glossary.......................................................................................................................................................145

Appendixes.................................................................................................................................................151
vi

Introduction to Principal Aquifers and to This Report

This report contains the major findings of national and regional assessments 

of the quality of groundwater in the Principal Aquifers of the United States. The 

Principal Aquifers, more than 60 in number, are regionally extensive aquifers that 

supply most of the groundwater pumped across the Nation for drinking water, 

irrigation, and other uses. The aquifers are composed of rocks and sediment 

such as unconsolidated sand and gravel, carbonate rock (limestone), granite, 

or volcanic rock. The aquifers extend across State boundaries and lie beneath 

diverse land uses. Water-quality conditions in the Principal Aquifers are summarized in this report and compared to selected national benchmarks, such as those 

for drinking-water quality.

This is one of a series of reports by the U.S. Geological Survey that present major 

findings for the Principal Aquifers. This report specifically focuses on waterquality conditions at the national scale and summarizes findings about groundwater quality from nine regional assessments of selected Principal Aquifers or 

groups of aquifers. Water-quality conditions in selected Principal Aquifers are 

discussed in greater detail in companion U.S. Geological Survey circulars to this 

report and in other reports listed in the references. Detailed technical information, data and analysis, sample collection, and analytical methodology, models, 

graphs, and maps that support the findings presented in this report, in addition to 

the companion reports in this series, can be accessed from the National WaterQuality Assessment Program (NAWQA) Web site (http://water.usgs.gov/nawqa).
Chapter 1

About 130 million people in the United States rely on groundwater for 

drinking water, and the need for high-quality drinking-water supplies 

becomes more urgent as our population grows. Although groundwater 

is a safe, reliable source of drinking water for millions of people nationwide, 

high concentrations of some chemical constituents can pose potential 

human-health concerns. Some of these contaminants come from the rocks 

and sediments of the aquifers themselves, and others are chemicals that we 

use in agriculture, industry, and day-to-day life. When groundwater supplies 

are contaminated, millions of dollars can be required for treatment so that the 

supplies can be usable. Contaminants in groundwater can also affect the health 

of our streams and valuable coastal waters. By knowing where contaminants 

occur in groundwater, what factors control contaminant concentrations, and 

what kinds of changes in groundwater quality might be expected in the future, 

we can ensure the availability and quality of this vital natural resource in 

the future.

Photographs: Chesapeake Bay Program; copyright istockphoto.com; copyright 1980 by W.H. Mullins 

and published with permission

Chapter 1: Overview of Major Findings 

and Implications
National Findings

1 Contaminants from geologic or manmade sources were a potential 

human-health concern in one of every five wells sampled in the parts 

of aquifers used for drinking water

Groundwater from 22 percent of sampled wells—more than one in five—contained at least 

one chemical constituent at a concentration greater than a U.S. Environmental Protection 

Agency Maximum Contaminant Level (MCL) or other human-health benchmark for concentrations in drinking water. Most of these contaminants were from geologic sources—for example, 

arsenic, manganese, radon, and uranium. Nitrate was the only constituent from manmade 

sources that exceeded its human-health benchmark in more than 1 percent of wells.

2 Differences in geology, hydrology, geochemistry, and chemical use explain how and why aquifer vulnerability and concentrations of 

contaminants vary across the Nation

The geologic and manmade sources of contaminants that are present, how groundwater 

moves through an aquifer, and geochemical conditions within aquifers all affect concentrations of contaminants in groundwater. Because these factors differ among Principal Aquifers, 

different contaminants occur more—or less—frequently in some aquifers than in others. An 

understanding of how geology, hydrology, geochemistry, and chemical use affect the concentrations of individual contaminants is essential to explaining how and why groundwater quality 

varies across the Nation. In-depth regional assessments, based on comprehensive sampling of 

6,600 wells and ancillary data, provided this understanding for the major contaminants in each 

Principal Aquifer and, in some cases, have allowed us to predict concentrations across wide 

areas. This information also can be used to assess aquifer vulnerability and design efficient and 

effective programs for monitoring the Nation’s groundwater resources.

3 Changes to groundwater flow have also altered groundwater quality

People’s use of water, through irrigation, pumping, artificial recharge, and drainage, has 

drastically changed how water moves through some aquifers. In some parts of the western 

United States, the amount of water that flows through aquifers has doubled, tripled, or 

increased by even more. Such large changes have affected contaminants from both manmade 

and geologic sources. Irrigation and pumping have made the deep parts of some aquifers, which 

are used for drinking water, more vulnerable to contamination by nitrate, pesticides, and other 

manmade chemicals. Irrigation and other sources of artificial recharge have increased concentrations of dissolved solids in some shallow aquifers in dry climates. Irrigation, pumping, and 

artificial recharge, by mixing waters of different chemistry, have sometimes had the unexpected 

consequence of releasing contaminants, such as uranium, selenium, or radium, from aquifer 

rocks and sediment into the groundwater.

Photographs from USGS

2 Water Quality in Principal Aquifers of the United States, 1991–2010
4 Our actions today are determining groundwater quality for decades 

to come

Groundwater quality changes slowly. However, indicators of human influence on groundwater quality are increasing across the Nation, even over the relatively short time periods of 

single decades. Concentrations of dissolved solids, chloride, and (or) nitrate in groundwater 

increased in two-thirds of groundwater well networks that were sampled at 10-year intervals 

between the early 1990s and 2010. People’s influence on groundwater quality also is apparent 

in the concentrations of nitrate, pesticides, and other manmade chemicals found in shallow 

groundwater beneath agricultural and urban land. Concentrations of these chemicals exceeded 

human-health benchmarks two to four times more frequently in shallow groundwater beneath 

agricultural and urban land than in groundwater from the deeper parts of aquifers currently 

used for drinking water. Over time, the changes that we see in shallow groundwater are likely 

to appear in the deeper parts of aquifers, as the shallow groundwater moves downward. 

This change in quality of deeper aquifers is a concern for the future because the restoration 

of groundwater supplies that have become contaminated is difficult, is costly, and can take 

decades. In parts of many aquifers, we are still seeing the effects of contaminant inputs from 

more than 30 years ago; similarly, our actions today are determining groundwater quality for 

decades to come.

Photographs: Left, USGS. Top to bottom, USGS; Scott Bauer,

U.S. Department of Agriculture, Agricultural Research Service (USDA, ARS);

courtesy of the Indiana Geological Survey

Gallons of groundwater pumped 

each day

80,000,000,000

People who use groundwater for 

drinking water

130,000,000

People who use groundwater from 

private wells for drinking water

43,000,000

Chemical analyses performed in 

this study

1,300,000

6,600 Wells sampled for this study

62 Principl Aquifers in the United States

Chapter 1: Overview of Major Findings and Implications 3

Groundwater—The Invisible and Vital Resource
Photograph by Kelly Warner, USGS

Photograph by Michael Rupert, USGS

Findings From the Regional Principal Aquifer Assessments

Groundwater is hundreds of feet below the land surface 

in many parts of the volcanic-rock aquifers, located in 

the northwestern United States and Hawaii, but the high 

permeability of these aquifers allows contaminants from 

agriculture and other manmade sources to reach the water 

table. Nitrate is the most common contaminant, but concentrations of several pesticides and volatile organic compounds 

also exceeded human-health benchmarks, especially on Oahu, 

Hawaii. Use of some of these chemicals has been banned for 

decades, yet they continue to be detected in groundwater.

USGS Circular 1359, available at http://pubs.usgs.gov/circ/1359/

One-sixth of the Nation’s population relies on groundwater from the sands and gravels of the glacial aquifer 

system, in the northern United States, for drinking water. 

Contaminants from geologic sources—including arsenic and 

manganese, in more than 10 percent of sampled wells—were 

present in some areas at concentrations of potential concern 

for human health, especially in oxygen-depleted groundwater, 

which is more common in this aquifer than in many others. 

Agriculture, especially in the upper Midwest, is a source of 

nitrate and pesticides to groundwater, but low-permeability 

soils and artificial drainage reduce the aquifer’s vulnerability 

to contamination in some areas.

USGS Circular 1352, available at http://pubs.usgs.gov/circ/1352/

Glacial Aquifer System

Western Volcanics

4 Water Quality in Principal Aquifers of the United States, 1991–2010

cir1360_ch03_glacialindex

Glacial 

aquifer system

cir1360_ch03_columbiasnakehawaii

Hawaiian

volcanicrock aquifers

Snake River 

Plain aquifers

Columbia 

Plateau

aquifers
Photograph by Susan Thiros, USGS

Photograph by Steve W. Krull, copyright istockphoto.com

Contaminants from geologic sources are more common 

in the Southwest basin-fill aquifers than in many other 

aquifers in the Nation because of the hot and dry climate, long 

groundwater flow paths, and types of rocks and sediments 

present. Irrigation and pumping have greatly altered natural 

flow systems in some areas, increasing concentrations of 

dissolved solids and carrying manmade contaminants down 

to shallow and deep groundwater. Arsenic, dissolved solids, 

and nitrate are some of the contaminants of concern for 

drinking-water quality; their concentrations in groundwater 

throughout the Southwest basin-fill aquifers can be predicted 

on the basis of geology, climate, hydrology, and other natural 

and manmade factors.

USGS Circular 1358, available at http://pubs.usgs.gov/circ/1358/

Groundwater in the deep sandstone layers of this aquifer 

system provides high-quality drinking water to the Front 

Range urban corridor of Colorado, but shallow groundwater 

quality has been degraded by irrigation and other human 

activities. Dissolved solids are relatively high in the shallow 

aquifer, and irrigation water has mobilized contaminants, such 

as uranium, from geologic sources. The shallow groundwater 

can migrate into deep aquifer layers, increasing the vulnerability of drinking-water supplies to contamination.

USGS Circular 1357, available at http://pubs.usgs.gov/circ/1357/

Denver Basin Aquifer System

Southwest Basin-Fill Aquifers

Chapter 1: Overview of Major Findings and Implications 5

cir1360_ch03_denver

Denver Basin

aquifer system 

cir1360_ch03_swpa

Basin and Range

basin-fill aquifers

California

Coastal

Basin

aquifers

Central Valley

aquifer system

Rio Grande

aquifer system
Photograph copyright istockphoto.com

Findings From the Regional Principal Aquifer Assessments

Geology largely determines where contaminants occur 

in these aquifers, which supply a large suburban and rural 

population in the eastern United States with drinking water. 

Radon, arsenic, and manganese occur in the crystalline-rock 

or siliciclastic (sandstone, siltstone, or shale) aquifers. The 

carbonate-rock aquifers, in contrast, are particularly susceptible to contamination from human activities at land surface 

because of karst (solution) features. Concentrations of nitrate 

in the carbonate-rock aquifers were among the highest in the 

Nation. Fecal-indicator bacteria were detected in half of the 

drinking-water sources sampled in the Piedmont, Blue Ridge, 

and Valley and Ridge aquifers.

USGS Circular 1354, available at http://pubs.usgs.gov/circ/1354/

Manmade contaminants—nitrate, pesticides, and volatile 

organic compounds—move readily through the sandy, 

permeable sediments of the Northern Atlantic Coastal Plain 

surficial aquifer system, located along the east coast of the 

United States; the areas of intense agriculture or dense population centers are particularly vulnerable. Radium from geologic 

sources is a concern because the natural acidity of the groundwater causes radium to be released from aquifer sediments 

into the groundwater. Groundwater from the shallow aquifer 

drains to streams and supplies nitrate that contributes to the 

ecological degradation of estuaries along the Atlantic coast.

USGS Circular 1353, available at http://pubs.usgs.gov/circ/1353/

Piedmont, Blue Ridge, and Valley and Ridge Aquifers

Northern Atlantic Coastal Plain Surficial Aquifer System

Photograph by Emily Nauman, Integration and Application Network, University 

of Maryland Center for Environmental Science (IAN, UMCES)

6 Water Quality in Principal Aquifers of the United States, 1991–2010

cir1360_ch03_northatlantic coastal plain

Northern Atlantic 

Coastal Plain 

aquifer system

cir1360_ch03_piedmont

Valley and Ridge

siliciclastic aquifers

Valley and Ridge

carbonate aquifers

Piedmont and Blue Ridge 

crystalline aquifers

Early

Mesozoic

basin

aquifers

Piedmont

and 

Blue Ridge

carbonate

aquifers
Photograph by Alan M. Cressler, USGS

These sandy aquifers in the southeastern and southcentral United States are pumped heavily for irrigation and 

public supply, but have few manmade contaminants. Manganese and arsenic, which are derived from the aquifer sediments, are the most common contaminants. Concentrations 

of nitrate are low despite large fertilizer applications because 

of low recharge rates and natural attenuation in the oxygendepleted groundwater. Pumping of supply wells has drawn 

deep, saline water up into the freshwater aquifer in some areas.

USGS Circular 1356, available at http://pubs.usgs.gov/circ/1356/

Groundwater in the Upper Floridan carbonate-rock 

aquifer, in the southeastern United States, has few contaminants from geologic sources and, in many areas, is protected 

from manmade contaminants by a low-permeability confining 

layer that inhibits downward flow. In some areas, where the 

confining unit is thin or absent, sinkholes and other karst 

features allow nitrate and pesticides from the land surface to 

move rapidly into and through the aquifer. Pumping of supply 

wells can enhance aquifer vulnerability to contamination by 

drawing shallow groundwater into the deep aquifer.

USGS Circular 1355, available at http://pubs.usgs.gov/circ/1355/

Mississipppi Embayment–Texas Coastal Uplands 

Aquifer System and Mississippi River Valley 

Alluvial Aquifer

Upper Floridan Aquifer and Overlying 

Surficial Aquifers

Chapter 1: Overview of Major Findings and Implications 7

Photograph by Jim Wark, Airphoto North America

Upper

Floridan

aquifer

Surficial

aquifers

Mississippi 

embayment

aquifer system 

Texas coastal

uplands aquifer

system 

Mississippi 

River Valley

alluvial aquifer 
Findings From the Regional Principal Aquifer Assessments

Photograph courtesy of John Charlton, Kansas Geological Survey

The thick, extensive sediments of the High Plains 

aquifer, in the central United States, supply one-third of the 

groundwater pumped for irrigation in the Nation and are used 

for public and private supplies. Recently recharged shallow 

groundwater, especially beneath irrigated cropland, has higher 

concentrations of dissolved solids and nitrate than deep 

groundwater that was recharged thousands of years ago. The 

aquifer has little natural capacity to attenuate nitrate because 

even deep, old groundwater commonly contains dissolved 

oxygen. High-capacity pumping wells with long screens 

can cause shallow and deep groundwater to mix, drawing 

manmade contaminants into deeper parts of the aquifer and 

altering groundwater geochemistry.

USGS Circular 1337, available at http://pubs.usgs.gov/circ/1337/

High Plains Aquifer

8 Water Quality in Principal Aquifers of the United States, 1991–2010

cir1360_ch03_highplainsindex

High

Plains

aquifer 
Chapter 2

The Principal Aquifers extend beneath vast and diverse areas of forests, 

plains, and deserts; they also lie beneath our agricultural lands and 

residential neighborhoods. How was the groundwater quality in such 

extensive and diverse areas assessed? The U.S. Geological Survey (USGS) 

National Water-Quality Assessment (NAWQA) Program used different types 

of groundwater studies, located across the United States, in which networks 

of wells were sampled for a comprehensive suite of water-quality constituents 

(appendix 1) using nationally consistent methods. A consistent study design 

means that results can be analyzed at multiple spatial scales, from the local 

scale of individual groundwater studies to the regional scale of Principal Aquifers (fig. 2–1) and, more broadly, to the national scale where NAWQA provides 

a unique perspective on the Nation’s groundwater quality. In addition to the 

chemical data, a wide range of information about the wells, the aquifer, and the 

surrounding environment was collected (see sidebar, NAWQA assessments use 

a wide range of geochemical data and site information, p. 16); these data are 

used to understand the factors that affect groundwater quality.

Chapter 2: Principal Aquifers and NAWQA 

Approach to Assessing Groundwater Quality

This chapter summarizes 

the study design used 

to investigate water 

quality in the Principal 

Aquifers.

Water samples were collected 

from domestic wells, publicsupply wells, and monitoring 

wells. More than 6,000 wells 

were sampled.
Several alluvial aquifers of limited areal extent 

are not listed or shown. Modified from 

U.S. Geological Survey.(1)

1

2

3

7

10

11

12

14

14

15

16

17

17

18

19

26

21

22

25

26

28 29

30w

30wc

30c

31

32

33

34

35

36

37

38

39

41

30w

17

22

2

32

7

30w

30wc

30c

39

21

21

31

31 31

38

29

25

14

10

11

23

35

30wc

Figure 2–1. The quality of groundwater in about 40 of the Nation’s Principal Aquifers is described in this circular, based on groundwater samples 

hydrogeology. The map shows the uppermost regional aquifer in an area; parts of some aquifers are buried beneath other aquifers or geologic units. 

and more than half of the water supplied by private domestic wells nationally.

10 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 2

10 Upper Floridan aquifer

1 Columbia Plateau basaltic-rock aquifers

2 Hawaiian volcanic-rock aquifers

3 Snake River Plain basaltic-rock aquifers

Basalt and volcanic-rock aquifers

4 New England crystalline-rock aquifers

5 Piedmont and Blue Ridge crystalline-rock aquifers

 6 Rocky Mountain Front Range crystalline-rock

 aquifers (not shown)

Crystalline-rock aquifers

8 Biscayne aquifer

7 Basin and Range carbonate-rock aquifers

9 Castle Hayne aquifer

11 Ordovician aquifers

Carbonate-rock aquifers

12 Ozark Plateaus aquifer system

13 Piedmont and Blue Ridge carbonate-rock aquifers

14 Silurian-Devonian aquifers

16 Edwards-Trinity aquifer system

Sandstone and carbonate-rock aquifers

17 Mississippian aquifer

15 Valley and Ridge carbonate-rock aquifers

25 Coastal lowlands aquifer system

Semiconsolidated sand and gravel aquifers

26 Mississippi embayment aquifer system

27 Northern Atlantic Coastal Plain aquifer system

28 Southeastern Coastal Plain aquifer system

29 Texas coastal uplands aquifer system

18 Cambrian-Ordovician aquifer system

Sandstone aquifers

19 Denver Basin aquifer system

20 Early Mesozoic basin aquifers

21 Lower Tertiary aquifers

22 Pennsylvanian aquifers

23 Valley and Ridge siliciclastic-rock aquifers

 24 Woodbine aquifer (not shown) 

Glacial aquifer system—Aquifers

 are discontinuous in area shown

30w Western glacial aquifers

30wc West central glacial aquifers

30c Central glacial aquifers

30e Eastern glacial aquifers

31 Basin and Range basin-fill aquifers

32 California Coastal Basin aquifers

33 Central Valley aquifer system

34 Columbia Plateau basin-fill aquifers

35 High Plains aquifer

36 Mississippi River Valley alluvial aquifer

37 Northern Rocky Mountain 

 Intermontane Basins aquifers

38 Rio Grande aquifer system

39 Snake River Plain basin-fill aquifers

40 Surficial aquifers 

Unconsolidated sand and gravel aquifers 

 (nonglacial)

41 Willamette Lowland basin-fill aquifers

4

5

8

9

13

17

20

23

27

30e

40

22

10

28

27

40

20

23

15

EXPLANATION

collected between 1991 and 2010. The aquifers are grouped here according to rock or sediment type into categories with broadly similar 

The Principal Aquifers shown here, which are included in this circular, provide about 90 percent of the groundwater pumped for public supply 

Chapter 2: Principal Aquifers and NAWQA Approach to Assessing Groundwater Quality 11
Table 2–1. The water-quality assessments summarized in this circular use data from three types 

of groundwater studies.

Characteristic

Type of groundwater study

Aquifer study Agricultural

land-use study

Urban 

land-use study

Study design objective

Assess quality of water 

in the parts of aquifers 

used for drinkingwater supply

Assess quality of shallow 

groundwater beneath 

agricultural land

Assess quality of shallow 

groundwater beneath 

urban land

Land use in study area Mixed Agricultural Residential 

and commercial

Number of studies 123 64 46

Total number of wells 3,669 1,793 1,158

Type of well

Mostly existing domestic 

wells; also some other 

types of supply wells

Mostly monitoring 

wells, installed for 

NAWQA

Mostly monitoring 

wells, installed for 

NAWQA

Number of wells per 

sampling network 

(typical)

20–30 20–30 20–30

Well selection method Random Random Random

Number of aquifers with 

studies of this type 41 22 22

Groundwater Studies

Groundwater studies are the building blocks of the Principal Aquifer water-quality assessments. There were three main types of studies, each with a different focus on groundwater 

quality and the natural and human-related factors that influence it (table 2–1).

Aquifer studies were designed to broadly assess water-quality conditions in the parts 

of aquifers used for drinking-water supply. For each study, 20 to 30 randomly located supply 

wells, mostly privately owned household (domestic) wells, were selected for sampling. The 

study areas were based on aquifer hydrogeology without regard to the overlying land use. 

This was the primary type of groundwater study used to assess water quality in the Principal 

Aquifers.

Agricultural and urban land-use studies were designed to characterize the quality of 

recently recharged groundwater in these land-use settings by sampling groundwater near the 

water table, mostly from shallow monitoring wells installed by the USGS. Most (86 percent) of 

the wells in land-use studies tapped groundwater from within 50 feet (ft) of the water table, and 

the median depth of the land-use study wells was 16 ft below the water table. Diverse types of 

agricultural areas were studied—areas with fields of wheat and other grains, mixed row crops 

and poultry, and rangeland. The areas sampled in urban settings primarily were residential, 

typically with low to medium population densities (300 to 5,600 people per square mile).

Results from more than 200 groundwater studies, each based on a sampling network of 

20 to 30 wells, are synthesized in this circular (fig. 2–2). The studies were conducted between 

1991 and 2010. Most wells were sampled only once during this time; for wells that were 

sampled repeatedly, data from the most recent samples were used to characterize the groundwater quality. More than 6,000 wells were sampled in total (fig. 2–2).

12 Water Quality in Principal Aquifers of the United States, 1991–2010

Groundwater studies, 

each based on a sampling 

network of 20 to 30 wells, 

are the building blocks 

of the Principal Aquifer 

assessments.
Chapter 2

!

Groundwater study type—

Symbol shows central 

location of sampling 

network of 20 to 

30 wells

Agricultural land-use study

Urban land-use study

Aquifer study

EXPLANATION

EXPLANATION

Groundwater study wells

Well from aquifer study

Well from agricultural 

 land-use study

Well from urban 

 land-use study

Well locations

Groundwater studies 

Figure 2–2. Each of the 233 groundwater studies (top map) in this water-quality assessment is based on a sampling network of 

20 to 30 wells. More than 6,000 wells (bottom map) were sampled in total. About half of the studies focused on the parts of aquifers 

used for drinking water (aquifer studies); the remainder were studies of shallow groundwater beneath agricultural or urban land 

(land-use studies).

Chapter 2: Principal Aquifers and NAWQA Approach to Assessing Groundwater Quality 13
Domestic well

Public-supply well Monitoring well

Two types of wells that supply drinking water 

were sampled: domestic (private) wells and 

public-supply wells. Domestic wells are usually 

shallower than public-supply wells and, therefore, 

pump water that is nearer to sources of manmade 

contaminants, such as fertilizers and pesticides, 

at the land surface. They commonly are located in 

rural areas and so are more likely than publicsupply wells to be vulnerable to contamination 

from agricultural chemicals.

Public-supply wells pump larger volumes 

of water than domestic wells, from deeper in 

the aquifer. They have larger pumps and longer 

screened intervals, and are pumped for longer 

periods of time than domestic wells. Because 

they are commonly located in suburban and urban 

areas, public-supply wells are more likely than 

domestic wells to be vulnerable to contamination 

from chemicals associated with urban activities; 

because they pump water from deeper in the 

aquifer, public-supply wells may be more likely 

to have high concentrations of constituents from 

geologic sources than domestic wells. The larger 

capture zones of public-supply wells also means 

that they are more vulnerable than domestic 

wells to manmade contamination from distant 

sources. If the amount of water withdrawn is 

large enough, it can change the flow direction 

and velocity of the groundwater, which can, in 

turn, affect the groundwater geochemistry of the 

constituents contained. Water from publicsupply wells is required to be tested by the 

well operator on a routine basis to help assure 

that the water provided to consumers meets 

Federal and State water-quality standards. 

Routine testing of water from domestic wells is 

not required. Homeowners are responsible for 

testing, maintenance, or treatment of the water 

from their domestic well.

Groundwater samples also were collected 

from wells that supply water for purposes other 

than drinking, such as irrigation, livestock, recreational facilities, and commercial or industrial 

uses. These wells can have pumping rates 

comparable to public-supply or domestic wells; 

they might be pumped every day or only during 

spring and summer. Depending on their depth, 

location, and pumping rate, these supply wells 

may be more or less vulnerable to contaminants 

associated with agriculture or urban activities.

What types of wells were sampled, and how might that affect the 

water quality measured?

14 Water Quality in Principal Aquifers of the United States, 1991–2010
cir1360_ch2_commonsidebar_anatomy ofawell

When pumped, groundwater flows through the 

screened interval and 

up and out of the well

Unsaturated

Saturated

Water table

Screen

Well bore

Water level

in well

Wellhead

Well casing

Pump

Grout

Gravel pack

Anatomy of a well

A well is simply a hole in the ground (well bore or borehole) 

from which water can be removed. The well bore is lined with 

a well casing, such as a pipe, to prevent the well bore from 

collapsing. The casing, along with a sealant (called grout), also 

prevents water from flowing into the well from the land surface 

or from parts of the aquifer where the water quality may be less 

desirable. The casing can be open at the bottom or perforated at 

a specific depth with a screen, to allow water to flow into the well 

where it can be pumped to the surface. Coarse sand or gravel 

(called sand pack or gravel pack) can be placed around the well 

screen to help improve the flow of water into the well. Some wells 

are cased only near the land surface, allowing water to flow into 

the well from nearly the entire length of the well bore.

Understanding study results

Important aspects of the NAWQA Principal Aquifer assessments:

•	 Water samples were collected at the wellhead prior to any treatment. They represent the quality of the groundwater 

resource but not necessarily the quality of tap water.

•	 The focus of the assessments is the condition of the total resource, including groundwater in a wide range of hydrologic 

and land-use settings across the Nation, rather than conditions at specific sites with known water-quality concerns.

•	 A wide range of constituents and properties are measured, including many that are not measured in programs that monitor 

water quality for compliance with drinking-water regulations. These data, along with ancillary information about the aquifer 

and surrounding environment (See sidebar, NAWQA assessments use a wide range of geochemical data and site information, p. 16), allow scientists to understand the sources and processes that affect groundwater quality.

•	 The assessments are guided by a nationally consistent study design, and all assessments use the same methods of 

sampling and analysis. Findings apply to water quality of a particular aquifer but also contribute to the larger picture of how 

and why water quality varies regionally and nationally. This consistent approach helps to determine if a water-quality issue 

is isolated or widespread. (See http://water.usgs.gov/nawqa/about.html for more information.)

•	 The assessments focused on aquifers used for water supply or on shallow groundwater that underlies a particular type of 

land use. Because NAWQA groundwater study areas do not cover the full spatial extent of the targeted Principal Aquifer, 

the findings might not represent the effects of the full range of geology, climate, and land use present.

•	 Analytical methods used by USGS chemists for assessments of water quality in Principal Aquifers are designed to measure 

constituents at as low a concentration as feasible. As a result, constituents frequently are detected at concentrations far 

below human-health benchmarks (see sidebar, Human-health benchmarks and other guidelines used in this assessment, 

p. 38). Low-level detections allow scientists to identify and evaluate emerging issues and to track contaminant levels 

over time.

Chapter 2

Chapter 2: Principal Aquifers and NAWQA Approach to Assessing Groundwater Quality 15
Chemists at the USGS National Water Quality Laboratory 

analyze groundwater samples using an array of sophisticated 

techniques.

Additional information (often called “ancillary information”) 

about the well and the surrounding environment complements 

the chemical data measured. This additional information often 

is key to making sense of the chemical data. For example, 

the information might be used to determine that shallow 

groundwater is more (or less) vulnerable to contamination 

than deep groundwater, that domestic wells are more (or less) 

vulnerable to contamination than public-supply wells, or that 

urban land use is associated with different types of groundwater 

contamination than is agricultural land use. Chemical data 

without accompanying ancillary data are much less useful for 

understanding factors that affect groundwater quality. 

NAWQA assessments use a wide range of geochemical data and site information

Constituents measured in samples from most wells

Constituent group Examples

Water-quality properties pH, specific conductance, dissolved 

oxygen, temperature

Major ions (filtered) Bromide, calcium, chloride, 

magnesium, sodium, sulfate

Trace elements (filtered) Arsenic, boron, iron, manganese, 

selenium, uranium

Nutrients (filtered) Ammonia, nitrate, phosphorus

Pesticides (filtered) Herbicides, insecticides, fungicides

Volatile organic compounds Solvents, gasoline hydrocarbons, 

refrigerants, trihalomethanes, 

fumigants

Organic carbon (filtered)

Additional constituents measured in samples from some wells

Constituent group Examples

Radionuclides Radon

Groundwater age tracers Tritium, chlorofluorocarbons

Stable isotopes Oxygen-18, hydrogen-2

Microorganisms Escherichia coli and total coliforms

Additional site information

Use of well

Well depth

Depth to water

Well-construction data

Principal Aquifer

Land-surface elevation at well

Land use within a 500-meter 

(1,640-foot) radius buffer

Estimates of nutrient inputs

Estimates of pesticide use

16 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 2

High Plains aquifer—Circular 1337

Glacial aquifer system—Circular 1352

Northern Atlantic Coastal Plain 

surficial aquifer system—Circular 1353

Piedmont, Blue Ridge, and Valley and 

Ridge aquifers—Circular 1354

Piedmont and Blue Ridge 

carbonate-rock aquifers

Piedmont and Blue Ridge 

crystalline-rock aquifers

Valley and Ridge siliciclasticrock aquifers

Valley and Ridge carbonaterock aquifers

Early Mesozoic basin aquifers 

Upper Floridan aquifer and overlying

surficial aquifers—Circular 1355

Mississippi embayment–Texas coastal 

uplands aquifer system—Circular 1356

Mississippi River Valley alluvial aquifer

Denver Basin aquifer system—Circular 1357

Southwest Principal Aquifers—Circular 1358

California Coastal Basin aquifers

Central Valley aquifer system

Basin and Range basin-fill aquifers

Rio Grande aquifer system 

Western Volcanics—Circular 1359

Hawaiian volcanic-rock aquifers

Snake River Plain basin-fill and 

basaltic-rock aquifers

Columbia Plateau basin-fill and 

basaltic-rock aquifers

In-depth Regional Assessments

Regional Assessments of Principal Aquifers

About half of the Principal Aquifers included in this circular received in-depth study as 

part of regional water-quality assessments (fig. 2–3). These aquifers included many of the 

most heavily used aquifers, supplying water for drinking and for irrigation across the Nation. 

Findings from the regional Principal Aquifer assessments are summarized in this circular and 

are fully described in nine companion circulars (http://water.usgs.gov/nawqa/studies/praq/). 

Figure 2–3. In-depth, regional-scale assessments of groundwater quality focus on the most heavily 

used Principal Aquifers in the Nation. Groundwater quality in these aquifers is described in nine 

U.S. Geological Survey circulars.

Chapter 2: Principal Aquifers and NAWQA Approach to Assessing Groundwater Quality 17
Decadal Trends

How is groundwater quality changing? To address this question, well networks in some 

groundwater study areas are being revisited at 10-year intervals for repeated sampling. Samples 

from 1,295 wells in 56 groundwater studies have been analyzed to date.(2, 3) About one-third 

of the studies were aquifer studies, and the rest were land-use studies. Well networks that were 

initially sampled from 1988 through 2000 were resampled 10 years later from 2001 through 

2010. The two sets of samples from each well network were compared, and the changes 

were analyzed to determine whether concentrations in the groundwater study area generally 

increased, decreased, or remained the same over the 10-year period. This approach aggregates 

local-scale changes in water quality at individual wells and provides a statistically based, broad 

overview of changes in water quality across the Nation. 

Photograph by Patrick C. Mills, USGS

18 Water Quality in Principal Aquifers of the United States, 1991–2010

Some wells are sampled 

repeatedly, at 10-year 

intervals, to monitor 

changes in groundwater 

quality.
An unconfined aquifer is bounded at its top by the 

water table, below which water fills all the pore spaces 

in the rock. Water from the land surface can move 

down into an unconfined aquifer.

A confining layer is a layer of material (commonly clay) 

through which water does not easily flow, creating a 

boundary between aquifers. 

A confined aquifer is bounded at its top by a confining 

layer. Water enters or “recharges” confined aquifers 

where the confining layer is not present. Where the 

confining layer is not continuous or is breached (for 

example, by a well), flow between the unconfined and 

confined aquifer can occur. 

The unsaturated zone is the area below the land 

surface and above an aquifer. In addition to soil, 

rocks, and air, it contains water from the land surface 

(such as rain) that is slowly moving downward to the 

water table of the aquifer. 

Confining layer

Confined aquifer

Unconfined aquifer

Unsaturated zone

Water table Wells

Surface water

Bedrock aquifer

Groundwater storage

and flow in fractures

Sedimentary aquifer

Groundwater storage 

and flow between

grains of sediment

Carbonate aquifer

Groundwater storage

and flow in solution

cavities or fractures

Screen

Anatomy of an aquifer

The water table is the upper surface of the

saturated zone. In the saturated zone, void spaces

are completely filled with water 

W

Chapter 3

hat is an aquifer? An aquifer is defined as saturated, permeable 

geologic material (rock or sediment) that will yield a useful quantity 

of water to a well (see sidebar, Anatomy of an aquifer, this page). 

Regionally extensive aquifers and aquifer systems(1) have been mapped as the 

Nation’s Principal Aquifers (fig. 2–1). Aquifers can be made up of a variety 

of geologic materials, such as sand, gravel, limestone, basalt, or sandstone. 

Aquifers commonly are characterized by their geologic materials (see sidebar, 

Principal Aquifer rock types, p. 20) because rock type is directly related to an 

aquifer’s capacity to transmit water—its permeability—and to other hydrogeologic characteristics. 

Chapter 3: Aquifers and Processes That Affect 

Groundwater Quality 

This chapter includes 

information about the 

Principal Aquifers, 

such as rock type, 

water use, and land 

use, and describes 

physical, chemical, and 

biological processes 

affecting groundwater 

quality.
Photographs from top to bottom: Clifford Voss, 

USGS; Sarah Flanagan, USGS; Suzanne Paschke, 

USGS; New Jersey Geological and Water Survey

Volcanic-rock aquifers are made up of 

basaltic lava flows, ash, and other volcanic rocks. 

In basaltic rocks, groundwater flows through pore 

spaces that have formed as the lava solidifed. 

Volcanic-rock aquifers are widespread in several 

western States and Hawaii. Thin-bedded basalt 

flows from the island of Hawaii are shown.

Crystalline-rock aquifers can be composed 

of granite, gneiss, schist, or other igneous and 

metamorphic rocks. These rocks are permeable 

only where they are fractured. Crystalline-rock 

aquifers extend beneath large areas of the eastern 

United States and are the only water-supply 

source in some areas. Rocks from the New 

England crystalline-rock aquifers in southern New 

Hampshire are shown; groundwater flowing from 

fractures has frozen on the outcrop.

Sandstone aquifers are rocks of cemented 

sandy deposits. Sandstone aquifers transmit water 

primarily along bedding planes and fractures, 

and they can be layered with other sedimentary 

rocks, such as siltstones and shales. Sandstone 

aquifers are present across the Nation, sometimes 

partly buried beneath other aquifers or geologic 

units. Sandstone layers from the Denver Basin 

aquifer system in Colorado are shown; these 

sandstones are geologically young and are not well 

consolidated.

Limestone and other sedimentary rocks made 

up of carbonate minerals form the carbonate-rock 

aquifers. Carbonate rocks are more soluble in 

groundwater than most other rocks, leading to 

the creation of cavities—caverns, conduits, and 

sinkholes—that can greatly enhance the rock’s 

ability to transmit water. Carbonate-rock aquifers 

are most extensive in the eastern United States. 

Limestone beds from a carbonate-rock aquifer in 

the Delaware Valley of New Jersey are shown.

Principal Aquifer rock types

The Nation’s Principal Aquifers are made up of a variety of geologic materials (rock types), ranging from hard, consolidated 

bedrock like basalt and granite to unconsolidated sand and gravel. The photographs show examples of these rock types, and the 

maps show the areal extent of aquifers of each rock type included in this circular.

20 Water Quality in Principal Aquifers of the United States, 1991–2010

cir1360_ch3_sidebarindex_volcanic.ai

cir1360_ch3_sidebarindex_crystalline.ai

cir1360_ch3_sidebarindex_sandstone.ai

cir1360_ch3_sidebarindex_carbonate.ai
Photographs from top to bottom: USGS; USGS; 

Brandon Curry, Illinois State Geological Survey; 

USGS

Chapter 3

Carbonate rocks and permeable sandstone 

rocks are interbedded (layered) in about equal 

amounts in the sandstone and carbonate-rock 

aquifers. The carbonate layers usually yield more 

water than the sandstones in these systems. 

Sandstone and carbonate-rock aquifers are 

in several parts of the United States. Springs 

discharge groundwater from the Edwards-Trinity 

aquifer to the river shown here in the Hill Country of 

south-central Texas.

Semiconsolidated sand aquifers are made 

up of variably cemented sand layered with silt, 

clay, and, in some places, carbonate rocks. These 

aquifers were deposited as sediments from rivers or 

in tidal water. They underlie the coastal plain along 

the Atlantic Ocean and Gulf of Mexico and form a 

thick wedge that dips toward the coast. Water flows 

through pores between sand grains, and aquifer 

productivity can be high. Shown are sediments from 

the Northern Altantic Coastal Plain in New Jersey.

The glacial aquifers contain unconsolidated 

sand, gravel, silt, and clay that was deposited 

primarily by meltwater from the continental ice 

sheets. They are widespread, though discontinuous, 

across the northern United States in river valleys, 

buried bedrock valleys, and broad lenses. Glacial 

aquifers also are buried beneath less permeable 

material in some places. Coarse-grained glacial 

deposits are widely used for public water supplies 

where present. A sand quarry in the glacial aquifer 

system in central Illinois is shown.

Unconsolidated sand and gravel aquifers

that are not of glacial origin fill low-lying areas 

between mountains in the west, form blanket-like 

sedimentary deposits in the High Plains, and overlie 

carbonate-rock aquifers in the southeast. These 

aquifers also underlie stream valleys throughout the 

United States. Their sediments were deposited by 

wind or water. The unconsolidated sand and gravel 

aquifers include large regional aquifers as well as 

important local aquifers. Deposits of coarse sand 

and gravel overlying fine-grained lakebed sediments near Great Salt Lake, Utah, are shown.

Chapter 3: Aquifers and Processes That Affect Groundwater Quality 21

circ1360_ch3_sidebarindex_glacial.ai

cir1360_ch3_sidebarindex_unconsolidated.ai

cir1360_ch3_sidebarindex_semiconsolidated.ai
1950

0

5

10

15

25

Pumping, in billion gallons per day

Year

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Groundwater pumping for public and domestic 

supply, and United States population, 1950–2005

Population, in millions of people

Water pumped for 

public supply

Water pumped by 

domestic wells

Population

20

0

50

100

150

200

250

300

EXPLANATION

Data from MacKichan,(161, 162) MacKichan and Kammerer,(163) Murray,(164)

Murray and Reeves,(165, 166) Solley and others,(167–170) Hutsen and others,(171)

Kenny and others,(4) and U.S. Census Bureau.(172)

Figure 3–1. The volume of groundwater pumped for public supply has quadrupled in the 

past 60 years, while the population of the United States has about doubled. The volume of 

groundwater pumped by privately owned household (domestic) wells has increased more 

slowly. Water pumped for public supply is used for drinking in homes, schools, and businesses, 

as well as for other uses in urban and suburban areas.

Water Use

Each day, about 80 billion gallons of water is pumped from the Nation’s aquifers. This 

water is used for drinking water, to irrigate crops and lawns, in industry, for aquaculture and 

livestock, in mining, to cool power plants, and for many other purposes.(4) Groundwater 

provides one-third of the water that is pumped by public-supply systems to provide the water 

used in homes, schools, and businesses in cities and towns. Use of groundwater for public 

supply has quadrupled during the past 60 years, as the population has steadily increased and 

cities and suburbs have expanded (fig. 3–1). Groundwater also is used for drinking water by 

rural homeowners with privately owned household (domestic) wells. Overall, about 130 million 

people currently get their drinking water from groundwater sources in the United States.

The Principal Aquifers assessed in this circular provide about 90 percent of the 

groundwater pumped for public supply, irrigation, and other uses nationally.(5, 6) The largest 

withdrawals are from the unconsolidated sand and gravel aquifers, and most of this water is 

used for irrigation (fig. 3–2). Public supply is the largest use of water pumped from the glacial 

aquifers and from most carbonate-rock, sandstone, and semiconsolidated sand aquifers. Water 

22 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 3

Piedmont and Blue Ridge carbonate

Rio Grande

High Plains

Basin and Range basin-fill

Columbia Plateau basin-fill

Early Mesozoic basin

Columbia Plateau basalt*

Central Valley

Mississippi River Valley Alluvial

New England crystalline

Basin and Range carbonate

Castle Hayne

Ozark Plateaus

Edwards-Trinity

Mississippian

Cambrian-Ordovician

Denver Basin sandstone

Lower Tertiary

Pennsylvanian

Valley and Ridge siliciclastic

Coastal lowlands

Mississippi embayment*

Northern Atlantic Coastal Plain

Southeastern Coastal Plain

Texas coastal uplands

California Coastal Basin

Northern Rocky Mountain Intermontane

Willamette Lowland

Ordovician

Piedmont and Blue Ridge crystalline*

Snake River Plain basin-fill

Valley and Ridge carbonate*

Snake River Plain basalt*

Silurian-Devonian

Principal Aquifer

Basalt and other

 volcanic-rock aquifers

Crystalline-rock

 aquifers

Carbonate-rock

 aquifers

Sandstone and carbonate-

 rock aquifers

Sandstone aquifers

Semiconsolidated

 sand aquifers

Glacial aquifer system

Unconsolidated

 sand and gravel

 aquifers

Hawaiian volcanic

Biscayne

Floridan*

Surficial

Undivided alluvial

Glacial

0 64 1614 18

Pumping, in billion gallons per day

2 8 10 12

Modified from Maupin and Barber(5) and Maupin and Arnold(6)

*Includes domestic use for an adjacent aquifer in this assessment.

**Domestic water use was not available for all aquifers.

Water use

EXPLANATION

 Public supply

 Irrigation

 Self-supplied industrial

 Domestic**

Figure 3–2. Irrigation, public supply, industrial use, and domestic use are major uses of the 

groundwater that is pumped from the Nation’s Principal Aquifers. Some of the largest withdrawals 

are from the unconsolidated sand and gravel aquifers in the western United States, and most of 

this water is used for irrigation. Altogether, the Principal Aquifers included in this study provide 

about 90 percent of the 80 billion gallons of groundwater that is pumped on average each day in the 

United States.

withdrawals for domestic supply are small compared with other withdrawals for public supply 

and irrigation in nearly all aquifers. However, about 43 million people, or 15 percent of the total 

population of the United States, rely on private wells for their drinking water. Many of these 

people live in rural areas where there is no other source of drinking water available. Groundwater quality is of particular concern for domestic well users because there are no regulations 

that require routine testing or treatment for contaminants in domestic wells in most States.

In parts of the Nation where the population is growing, the demand for water is increasing. 

Groundwater will become an increasingly important water source, especially in areas where 

supplies of water withdrawals from lakes, rivers, and reservoirs are limited. For example, new 

communities in the Denver Basin area that are without available surface water rely on groundwater sources, and withdrawals from the deep sandstone aquifers have tracked population 

growth.(7) In the upper Midwest, new water withdrawals from the Great Lakes, which supply 

water to large population centers, are limited to communities within the watershed; suburban 

communities outside the watershed rely on groundwater supplies to support their growing water 

demand.(8) Groundwater sources of sufficient quality to meet these existing and future water 

needs are critical for economic development and human health. 

Chapter 3: Aquifers and Processes That Affect Groundwater Quality 23
circ1353ch03_fig 03

Water

Land use

Mixed

Urban

Agricultural

Undeveloped

Data are from Homer and others(173) and U.S. Geological Survey(174)

EXPLANATION

Land Use

The Principal Aquifers lie underneath diverse types of land use. These land uses can affect 

the quality of water that infiltrates from the land surface and recharges groundwater. To study 

the effects of human activities on groundwater, NAWQA studies of shallow groundwater were 

located in areas of agricultural and urban land uses; these types of land use can result in large 

alterations of the land surface. Nationally, agricultural land use is most common in the central 

United States, and urban land use is most common in the East (fig. 3–3). However, parts of all 

aquifers across the Nation are overlain by some agricultural and urban land use (fig. 3–4).

Land use changes with time and these changes can affect water quality (fig. 3–5). 

Nationally, the amount of developed land in the United States increased by nearly 43 million 

acres, or 60 percent, between 1982 and 2010, mostly from conversion of forest and agricultural 

land; this increase is an area roughly the size of Oklahoma.(9) Chemical use, waste disposal, and 

irrigation that can accompany the new development in urban and suburban areas are sources 

of contaminants to the underlying aquifer. When suburbs and cities expand into cropland, 

fertilizers and pesticides used in agriculture can be replaced with chemicals associated with 

residential activities, such as volatile organic compounds (VOCs) or deicing chemicals, as 

potential contaminants to underlying groundwater.

Figure 3–3. Areas of agricultural and urban land were the focus of groundwater studies to investigate the effects of 

human activities on groundwater quality.

24 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 3

Piedmont and Blue Ridge carbonate

Rio Grande

High Plains

Basin and Range basin-fill

Columbia Plateau basin-fill

Early Mesozoic basin

Columbia Plateau basalt

Central Valley

Mississippi River Valley Alluvial

New England crystalline

Basin and Range carbonate

Biscayne

Castle Hayne

Floridan

Ozark Plateaus

Edwards-Trinity

Mississippian

Cambrian-Ordovician

Denver Basin sandstone

Lower Tertiary

Pennsylvanian

Valley and Ridge siliciclastic

Coastal lowlands

Mississippi Embayment

Northern Atlantic Coastal Plain

Southeastern Coastal Plain

Texas coastal uplands

Glacial

California Coastal Basin

Northern Rocky Mountain Intermontane

Surficial

Willamette Lowland

Ordovician

Piedmont and Blue Ridge crystalline

Snake River Plain basin-fill

Valley and Ridge carbonate

Snake River Plain basalt

Silurian-Devonian

0 20 40 60 80 100

Basalt and other

 volcanic-rock aquifers

Crystalline-rock

 aquifers

Sandstone and carbonate-

 rock aquifers

Sandstone aquifers

Semiconsolidated

 sand aquifers

Unconsolidated

 sand and gravel

 aquifers

Carbonate-rock

 aquifers

Glacial aquifer system

Principal Aquifer

Percentage of area overlying aquifer

Land use, in percentage of land area 

overlying the aquifer

Land Use

EXPLANATION

 Agricultural

 Urban

 Undeveloped 

and water

Data are from Homer and others(173) and U.S. Geological Survey(174)

Figure 3–5. Changes in land use from undeveloped or agricultural to urban or suburban can be 

accompanied by changes in the underlying groundwater quality. For example, urban expansion in 

Las Vegas, Nevada, increased the population from 273,000 people in 1972 to more than 2 million in 

2010. Effects on the quality of shallow groundwater include higher concentrations of nitrate and 

dissolved solids and more frequent detections of pesticides and VOCs than in shallow groundwater 

beneath undeveloped areas. Areas of irrigated vegetation are shown in red and impervious 

surfaces, such as rooftops and roads, are shown in gray on these Landsat satellite images. 

Figure 3–4. All aquifers are 

overlain by some areas of 

agricultural and urban land 

use. Many of the potential 

sources of manmade 

contaminants to groundwater 

are associated with these 

broad land-use categories. 

Areas are shown in figure 3–3.

Chapter 3: Aquifers and Processes That Affect Groundwater Quality 25

Changing land use in the Las Vegas Valley, Nevada

USGS Earth Explorer by the NASA/Goddard Space Flight Center
Saltwater

Shallow

monitoring 

well

Water

table

Confined aquifers

Publicsupply wells Coastal estuary

Freshwater-saltwater interface

Unconfined aquifer

Public-supply well

Confining units

Atlantic Ocean

Domestic well

NOT TO SCALE

EXPLANATION

Groundwater

 flow direction

Recharge

Discharge

Northern Atlantic Coastal Plain surficial aquifer system

Figure 3–6. Groundwater flows through aquifers from recharge areas to discharge areas. Where precipitation is abundant and aquifers are shallow 

In dry climates, such as in the Denver Basin aquifer system (right), recharge may occur primarily at mountain fronts or from leakage from streams. 

Groundwater Flow

Groundwater is not static but moves through aquifers, and the paths that it takes and 

rates at which it flows affect water quality. On a broad scale, groundwater flows from areas of 

recharge, where it enters the aquifer, to discharge areas, where it leaves the aquifer (fig. 3–6). 

The driving force behind groundwater flow is hydraulic head—a combination of elevation and 

water pressure.(10) Natural groundwater recharge occurs wherever rain and snowmelt percolate 

through the unsaturated zone to reach the water table. Recharge is less than total precipitation 

because some water from rainfall and snowmelt runs off directly to streams or returns to the 

26 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 3

Unconfined aquifer

Confined aquifer

Underlying bedrock

Denver

Public-supply

wells

Monitoring

well

Domestic

wells

Water

table

Confining units

NOT TO SCALE

Alluvial

aquifer

South Platte River

Denver Basin aquifer system

EXPLANATION

Groundwater

 flow direction

Recharge

Discharge

and unconfined, such as in the Northern Atlantic Coastal Plain surficial aquifer system (left), recharge typically occurs across the landscape. 

Discharge from aquifers can be to streams, coastal waters, pumping wells, or directly to plants or the atmosphere where the water table is shallow.

atmosphere through evapotranspiration. Some aquifers also are recharged by flow across 

confining layers, by inflow from adjacent aquifers, or by seepage from streams or lakes. Water 

discharges from an aquifer when it flows into a stream, to a pumping well, or to the ocean, or 

when it is evaporated at the land surface. A groundwater flow path is the route that the water 

takes through the aquifer from recharge to discharge. Contaminants from manmade or geologic 

sources in the area where the water is recharged (the capture zone) and in the aquifer along 

the groundwater flow path can affect the quality of the groundwater pumped from a well or 

discharged to a stream.

Chapter 3: Aquifers and Processes That Affect Groundwater Quality 27
Groundwater moves slowly—a flow rate of 1 foot per day is fast for groundwater, and flow 

rates can be as low as 1 foot per year or 1 foot per decade.(11) Consequently, it can take tens, 

hundreds, or even thousands of years for groundwater to travel through aquifers. The length of 

time that water remains in an aquifer before it discharges is called the groundwater residence 

time. Groundwater age, a related concept, is the time that has elapsed since groundwater was 

recharged. Groundwater that is deep and far from the aquifer’s recharge area typically is older 

than groundwater that is shallow and near the recharge area. Consequently, all aquifers contain 

groundwater of different ages (fig. 3–7). Flow rates and residence times depend on the aquifer’s 

size, shape, permeability, recharge rates, and hydraulic-head gradients.

Knowledge of how water moves through aquifers can help us interpret some of the 

groundwater-quality characteristics that we observe. For example, older groundwater that 

has traveled a long distance through an aquifer has had more time to interact with the aquifer 

rocks and sediments than younger groundwater and is likely to have higher concentrations of 

constituents from geologic sources as a result. In contrast, younger or shallower groundwater 

is more likely than older or deeper groundwater to have contaminants from manmade sources. 

Young groundwater in aquifers with low recharge rates also can have relatively high concentrations of constituents from geologic sources because there has been little dilution of these 

constituents in the unsaturated zone as compared with aquifers with high recharge rates in areas 

of abundant precipitation. Human activities that change how water moves through aquifers, 

such as pumping (which removes water from aquifers) and irrigation (which adds water to 

aquifers), can affect groundwater quality by mixing older and younger water or otherwise 

changing the distribution of chemical constituents in groundwater.

28 Water Quality in Principal Aquifers of the United States, 1991–2010

Groundwater moves 

slowly and can take 

tens, hundreds, or even 

thousands of years to travel 

through aquifers.

Groundwater that is deep 

and far from the aquifer 

recharge area typically is 

older than groundwater 

that is shallow and near the 

recharge area.
Chapter 3

Figure 3–7. Groundwater can be thousands of years old in some confined aquifers and in thick unconfined aquifers where 

recharge rates are low, such as the High Plains aquifer, Upper Floridan aquifer, and the Cambrian-Ordovician aquifer system. 

Groundwater is younger—often only a few decades old—in shallow, unconfined aquifers with high recharge rates, such as the 

glacial aquifer system and the uppermost layers of the Northern Atlantic Coastal Plain aquifer system. Old groundwater may be 

more likely to have contaminants from geologic sources because it has had longer to react with aquifer rocks and minerals, and 

may be less likely to have contaminants from recent manmade sources.

50

50 100 150

40

30

20

10

0

50

40

30

20

10

0

0

0

0 40 80 120 130

Young

Young

Young

Old

Old

Central Valley aquifer, 

California

Glacial aquifer 

system, Minnesota

Cambrian-Ordovician aquifer 

system, Wisconsin

Upper Floridan aquifer, Florida

Modified from, upper left clockwise: Delin and others(175) and Böhlke and others; (62)

Klump and others; (176) Dunkle and others(177) and Böhlke and Denver;(178) Plummer and Sprinkle;(68) 

McMahon and others;(56) Burow and others(96)

Depth in aquifer, in feet

Depth in aquifer, in feet

Age, in years

Age, in years

Age, in years

Age, in years

Age, in years

0 20 40 60

0

0 0 10 20 30 10 20 30

Groundwater age at selected locations in Principal Aquifers

35,000

30,000

20,000

10,000

35,000

30,000

20,000

10,000

0

0

Old

High Plains aquifer, Nebraska

Age, in years

20,000

15,000

10,000

5,000

200 400 600

Depth in aquifer, in feet Distance from recharge 

area, in miles

Distance from recharge 

area, in miles

Depth in aquifer, in feet

40

30

20

10

0

Northern Atlantic Coastal 

Plain aquifer system, Maryland

Chapter 3: Aquifers and Processes That Affect Groundwater Quality 29
Recharge rates

Natural groundwater recharge rates vary among and within 

Principal Aquifers because of regional differences in climate, soil 

type, vegetation, and topography. In general, recharge rates are 

higher where there is more rain and snow and are lower in hotter 

climates. Aquifers in the East, Northwest, and mountainous regions 

of the western United States have relatively high natural recharge 

rates compared with aquifers in the High Plains and southwestern 

United States. Recharge rate is one of the factors that influence the 

rate at which groundwater flows through an aquifer—groundwater 

moves very slowly through aquifers that receive only a few inches 

per year of recharge. Groundwater recharge can be greatly 

increased by human activities such as irrigation.

Irrigation-dominated and natural recharge rates

0

10

20

30

40

Recharge rate, in inches per year

Mean annual precipitation, in inches per year

Natural recharge

0 10 20 30 40 50

Irrigationdominated

recharge

Modified from McMahon and others(200)

0 to 0.5

> 0.5 to 1

> 1 to 5

> 10 to 15

> 15 to 20

> 5 to 10

> 20

Recharge rate, in 

inches per year

Natural groundwater recharge rates

Modified from Wolock(126)

EXPLANATION

Natural recharge rates increase as 

precipitation increases, but irrigation 

can greatly augment natural recharge in 

dry areas.

Rates of natural groundwater recharge 

vary more than tenfold nationally, 

primarily because of differences in 

precipitation and climate.

30 Water Quality in Principal Aquifers of the United States, 1991–2010
Table 3–1. Definitions of geochemical conditions and processes

Geochemical conditions

Alkalinity The capacity of the dissolved substances in water to react with and neutralize acid. Bicarbonate is an 

example of a dissolved substance that contributes to alkalinity.

pH Concentration of hydrogen ion in a solution, a measure of the acidity of the solution. Water with low pH 

(less than 7) is acidic and water with high pH (greater than 7) is alkaline; a pH of 7 is neutral.

Redox conditions Defined by the dominant type of redox reaction occurring in the water. Water is “oxic” when it contains 

dissolved oxygen and “anoxic” when dissolved oxygen is absent.

Geochemical processes

Biodegradation Transformation of a substance into new compounds through biochemical reactions or the actions of 

microorganisms such as bacteria.

Dissolution and precipitation

Processes by which minerals, or parts of minerals, pass into solution to become dissolved substances 

(dissolution) or come out of solution to form solids (precipitation). Halite (rock salt) is an example of a 

mineral that is easily dissolved; quartz (sand) is an example of a mineral that is not easily dissolved.

Reduction and oxidation 

(redox)

Chemical reactions that involve the transfer of electrons from one chemical species to another, 

resulting in a change in the valence state of the species. Redox processes in groundwater often 

are microbially facilitated.

Sorption and desorption

Processes by which dissolved substances such as ions become attached (sorption) or detached (desorption) 

to the surfaces of solid material. The actions of a household water softener are an example of sorption 

and desorption reactions, in which calcium and magnesium ions that cause hardness in water are 

exchanged for sodium. 

Chapter 3

Geochemical Conditions and Processes 

Geochemical conditions—pH, alkalinity, and reduction-oxidation (redox)—strongly 

affect the mobility and persistence of many chemical constituents in groundwater, including 

some that are of concern for human health. Geochemical conditions, together with constituent 

properties, control the reaction processes—dissolution and precipitation, sorption and desorption, redox processes, and biodegradation (table 3–1)—that affect constituent concentrations in 

groundwater. Geochemical conditions determine whether chemical constituents from geologic 

sources are released from the aquifer rocks and sediments or remain immobile in aquifer solids. 

Geochemical conditions also determine whether constituents from manmade sources travel 

with the groundwater, react with the aquifer material, or degrade into other chemicals along the 

groundwater flow path.

Groundwater pH can affect the concentrations of many trace elements because pH 

influences the sorption characteristics of aquifer rocks and sediments and, along with redox 

conditions, controls the solubility of some minerals. Trace elements that commonly exist as 

positively charged ions, including cadmium, cobalt, copper, lead, nickel, and zinc, are more 

likely to sorb onto aquifer rocks and sediments at higher pH. This is because, as pH increases, 

the surface charge of the metal oxide coatings and clay minerals in aquifer rocks and sediments 

becomes more negative, attracting the positively charged trace elements and decreasing their 

concentrations in groundwater.(12) Conversely, negatively charged trace elements, such as 

arsenic, chromium, selenium, and molybdenum, are more likely to be desorbed from aquifer 

rocks and sediments as pH increases, resulting in higher concentrations of these trace elements 

in groundwater.

Chapter 3: Aquifers and Processes That Affect Groundwater Quality 31
The pH of groundwater varies among aquifer rock types (fig. 3–8). Rates and types of 

weathering processes—chemical reactions that dissolve or otherwise break down rock-forming 

minerals—vary among rock types and can alter pH. Natural waters that are in contact with the 

atmosphere and biological activity are slightly acidic because they contain dissolved carbon 

dioxide. Dissolution of carbonate minerals consumes dissolved carbon dioxide and results 

in an increase in the pH of the water. Thus, groundwater in the carbonate-rock aquifers and 

unconsolidated aquifers with carbonate sediments commonly has near-neutral or alkaline pH 

(fig. 3–8). Weathering of silicate minerals, which are major components of many rock types 

and sediments, also consumes acidity, but silicate mineral weathering proceeds more slowly 

than carbonate dissolution. Weathering of silicate minerals in source rocks and in aquifers 

with long groundwater residence times is one reason why alkaline groundwater is prevalent in 

the unconsolidated sand and gravel aquifers and volcanic-rock aquifers in the western United 

States. In contrast, aquifers with abundant quartz sand, such as parts of the semiconsolidated 

sand aquifers of the Atlantic and Gulf of Mexico coastal plains, are relatively resistant to 

weathering reactions that neutralize acidity and more commonly have acidic groundwater than 

aquifers of other rock types.

32 Water Quality in Principal Aquifers of the United States, 1991–2010

Figure 3–8. Alkaline groundwater is more common in the carbonate-rock and sandstone aquifers where weathering reactions 

increase pH than in aquifers of other rock types. Acidic groundwater is especially prevalent in the semiconsolidated sand aquifers 

of the Atlantic and Gulf of Mexico coastal plains; these aquifers have little capacity to reduce the acidity of precipitation through 

weathering reactions. pH is an important control on the sorption and dissolution processes that affect the concentrations of some 

contaminants, such as arsenic, in groundwater.

More

alkaline More

acidic

5.5 or less

> 5.5 to 6.5

> 6.5 to 7.5

> 7.5 to 8.5

> 8.5

EXPLANATION

pH, in percentage of wells

Sandstone and 

carbonate-rock

Crystalline-rock Unconsolidated

sand and gravel

Semiconsolidated 

sand 

Glacial

Sandstone 

Carbonate-rock

pH by

Principal Aquifer rock type

Basaltic and other 

volcanic-rock
Chapter 3

How do redox reactions work?

Reduction/oxidation (redox) processes require one chemical species that donates electrons and another chemical species that 

accepts those electrons. As a chemical species donates electrons it is “oxidized,” and as the other species accepts electrons it is 

“reduced.” Redox processes typically are facilitated by microbes (bacteria), which use the energy produced by the processes. In 

groundwater, organic carbon is the most common electron donor. If dissolved oxygen is present, it is the preferred electron acceptor 

because reduction of dissolved oxygen produces more energy than reduction of other chemical species that commonly occur in 

groundwater. The atmosphere is the source of the dissolved oxygen, so the redox conditions in an aquifer near where recharge occurs 

usually are oxic (defined here as having a concentration of dissolved oxygen of at least 0.5 mg/L).

From a water quality perspective, denitrification—the microbially driven reduction of nitrate (NO3

-

) to nitrogen gas (N2) under anoxic 

conditions—is one of the most important redox processes that occurs in groundwater. Nitrate is a concern for human health and, where 

it discharges to surface water, can impair aquatic communities. Conversion of nitrate by denitrification to harmless nitrogen gas, the 

same gas that we breathe in the atmosphere, is the primary way that nitrate is removed from water. Similar to other redox processes, 

the reduction of nitrate is paired with the oxidation of some other chemical, such as organic carbon, which also must be present for 

denitrification to occur. Bicarbonate and carbon dioxide also are formed and raise the pH (make the water more alkaline).

Oxygen Nitrate Manganese Iron Sulfate Carbon dioxide

Oxic Anoxic

Oxic Mildly

reducing

Strongly

reducing

Very strongly

reducing

Order of consumption of electron acceptors

Redox conditions are important for groundwater quality because many chemical constituents can exist in several oxidation states that behave differently in aquifers. The redox condition 

of the groundwater—whether oxic (oxygen present) or anoxic (oxygen absent)—strongly 

influences the oxidation state of a chemical in the groundwater. Redox conditions influence 

sorption characteristics of aquifer materials because metal oxide coatings on rock and sediment 

surfaces, to which dissolved ions sorb, are stable under oxic conditions but can dissolve under 

anoxic conditions. Redox conditions influence the degradation processes that can transform 

contaminants, including nitrate and some manmade contaminants such as solvents and some 

pesticides.(13, 14)

Chapter 3: Aquifers and Processes That Affect Groundwater Quality 33

Older groundwater is 

more likely to be anoxic 

than younger groundwater 

because there has been 

more time for chemical 

reactions that consume 

dissolved oxygen to occur 

in older groundwater.
Redox conditions vary among aquifers and along groundwater flow paths because of 

differences in the chemical compositions of aquifer rocks and sediments and differences in 

groundwater age. Oxic conditions are dominant in the unconsolidated sand and gravel and the 

basaltic-rock aquifers, which are found mostly in the western United States (fig. 3–9); organic 

carbon and other electron donors that would consume the dissolved oxygen in recharging 

groundwater are typically not abundant in these aquifers(15) (see sidebar, How do redox 

conditions work?, p. 33). Oxic conditions also are prevalent in the crystalline-rock aquifers 

and the layered sandstone and carbonate aquifers, which are mostly in the eastern and central 

United States. Anoxic conditions are more common in the glacial, sandstone, carbonate-rock, 

and semiconsolidated coastal plain aquifers, which are mostly in the East. Wood fragments 

in glacial sediments or the remains of marine life in sedimentary rocks are examples of the 

organic-carbon-rich material that is likely to be more abundant in these aquifers and that 

depletes the oxygen dissolved in groundwater recharge.

In most aquifers, older groundwater is more likely to be anoxic than younger groundwater 

because there has been more time for chemical reactions that consume dissolved oxygen to 

occur in older groundwater. However, redox conditions can be quite variable across short 

distances because of small-scale variability in aquifers—the irregular distribution of organicrich layers or the presence of reduced minerals along fractures, for example. Differences in 

the environments and soils in the recharge area also can cause small-scale differences in redox 

conditions that are carried with the groundwater as it travels through the aquifer.

34 Water Quality in Principal Aquifers of the United States, 1991–2010

Figure 3–9. Groundwater is predominantly oxic in the volcanic-rock and unconsolidated sand and gravel aquifers, which are found 

mostly in the western United States. Anoxic conditions are more common in the glacial aquifer system and in aquifers of several other 

rock types that are found mostly in the North and East. These differences in redox conditions can affect the persistence of some 

contaminants, including nitrate and some pesticides and volatile organic compounds.

circ1360_ch03_fig9_mapredox-pies.ai

Less oxic

Oxic

Anoxic

Mixed

EXPLANATION

Redox condition, in percentage of wells

Sandstone and 

carbonate-rock

Basaltic and other 

volcanic-rock

Crystalline-rock

Unconsolidated sand 

and gravel

Semiconsolidated 

sand 

Glacial

Sandstone 

Carbonate-rock

Redox conditions by

More oxic Principal Aquifer rock type
Chapter 4

cir1360_ch04fig01

0 10 20 30 40 50

Exceedances of human-health 

benchmarks by one or 

more contaminants

Any contaminant

Contaminants

from geologic

sources*

Contaminants

from human

sources

* The higher of two proposed MCLs was used for radon.

Percentage of wells

Percentage of samples

5%

17%

22%

Photograph by Marc Dufresne, copyright istockphoto.com

Figure 4–1. Although most of the wells sampled in parts of 

aquifers used for drinking water had no constituent exceeding a 

human-health benchmark, 22 percent of wells had at least one 

contaminant whose concentration exceeded its benchmark. 

Contaminants from geologic sources accounted for most of 

these exceedances.

I

s this water suitable for human consumption? This important and frequently 

asked question can be addressed by comparing the chemical constituents 

of the water to human-health benchmarks—guidelines and standards for 

concentrations in drinking water that are considered protective of human health 

(see sidebar, Human-health benchmarks and other guidelines used in this 

assessment, p. 38). Nationally, nearly 80 percent of the 3,700 wells sampled in 

drinking-water aquifers had concentrations of measured chemical constituents 

less than human-health benchmarks. About 20 percent of wells, however, had at 

least one contaminant present at a concentration greater than the human-health 

benchmark for that contaminant (fig. 4–1; see sidebar, What is a contaminant?, 

p. 38). Contaminants from geologic sources were responsible for 78 percent of 

the concentrations that exceeded a human-health benchmark (fig. 4–2).

Chapter 4: The Quality of Groundwater 

Resources For Drinking and Other Uses 

This chapter identifies 

constituents that were 

detected at concentrations greater than 

human-health benchmarks and describes 

other aspects of water 

quality that can affect 

the use of the water.
circ1360_ch04_fig02_natlmapexcbypasus.ai

Early Mesozoic

basin

Valley and Ridge

carbonate

Valley and Ridge

siliciclastic

Castle Hayne

New England

crystalline

Biscayne

Upper

Floridan

Mississippian

Ordovician

CambrianOrdovician

Ozark

Plateaus

Coastal

lowlands

Northern Atlantic

Coastal Plain surficial

Mississippi

Rver Valley

Alluvial

Denver

Basin

Lower

Tertiary

EdwardsTrinity

Southeastern

Coastal Plain

Texas

coastal

uplands

Glacial

Surficial

Basin and

Range

basin-fill

California

Coastal

Basin

Central

Valley

Willamette

Basin and

Range

carbonate

Columbia

Plateau

basin-fill

Snake River

Plain

Snake River basalt

Plain

basin-fill

Columbia

Plateau

basalt

Rio Grande

High PlainsHigh Plains Northern

Rocky Mountain

Intermontane

Mississippi

embayment

SilurianDevonian

Pennsylvanian

Hawaiian

volcanic

Piedmont and Blue Ridge

crystalline

Glacial

None

From geologic sources

From human sources

Percentage of wells with one or more contaminant 

at a concentration greater than a human-health benchmark

EXPLANATION

Exceedances of human-health benchmarks by one or more contaminants

All aquifers

17% 5%

78%

Figure 4–2. Contaminants from geologic sources exceeded human-health benchmarks more frequently than contaminants from 

manmade sources in most Principal Aquifers. In some aquifers, including the Ozark Plateaus, Snake River Plain, Edwards-Trinity, and 

Rio Grande aquifers or aquifer systems, the only contaminants greater than benchmarks were from geologic sources. Contaminants 

from manmade sources were responsible for larger proportions of benchmark exceedances than contaminants from natural sources in 

a few aquifers overlain by intensive agriculture and (or) urban development, such as the Hawaiian volcanic-rock aquifers, the Biscayne 

aquifer in Florida, and the Northern Atlantic Coastal Plain surficial aquifer system along the east coast. (Wells with exceedances of both 

geologic and manmade contaminants are counted in both categories and are proportionately reduced so that values add to 100.)

36 Water Quality in Principal Aquifers of the United States, 1991–2010
Table 4–1. The constituents that most frequently exceeded human-health benchmarks are 11 contaminants from geologic sources 

and 5 from human sources. These 16 contaminants accounted for 98 percent of all concentrations greater than benchmarks measured 

in groundwater samples from the parts of aquifers used for drinking water.

[See appendix 2 for contaminants in individual Principal Aquifers. AMCL, Alternative Maximum Contaminant Level, proposed for radon;(192) MCL, 

U.S. Environmental Protection Agency Maximum Contaminant Level;(60) HBSL, U.S. Geological Survey Health-Based Screening Level, current as of 

April 2012;(194, 195) pCi/L, picocurie per liter; μg/L, microgram per liter; mg/L, milligram per liter]

Contaminant

Human-health benchmark

Number of 

wells sampled

Percentage of 

samples with 

concentrations 

greater than the 

benchmark

Number of 

Principal Aquifers 

with one or more 

exceedances of the 

Value Type benchmark

 Contaminants from geologic sources

Manganese 300 μg/L HBSL 3,662 6.9 25 of 41

Arsenic 10 μg/L MCL 3,074 6.7 20 of 37

Radon *4,000 pCi/L Proposed AMCL 3,120

3.6

15 of 41

(300 pCi/L) (Proposed MCL) (62)

Strontium 4,000 μg/L HBSL 1,956 1.7 10 of 29

Uranium 30 μg/L MCL 3,258 1.6 12 of 37

Fluoride 4 mg/L MCL 3,655 <1 8 of 41

Molybdenum 40 μg/L HBSL 3,036 <1 8 of 37

Lead 15 μg/L Action level 3,035 <1 3 of 37

Antimony 6 μg/L MCL 3,026 <1 4 of 37

Selenium 50 μg/L MCL 3,036 <1 4 of 37

Zinc 2,000 μg/L HBSL 2,979 <1 3 of 37

Any Various Various 3,669 16 35 of 41

Contaminants from human sources

Nitrate 10 mg/L as N MCL 3,621 4.1 21 of 41

Dieldrin †0.002 μg/L HBSL 3,553 <1 14 of 41

Perchloroethene 5 μg/L MCL 3,272 <1 5 of 41

Trichloroethene 5 μg/L MCL 3,322 <1 5 of 41

1,2-Dibromo3-chloropropane (DBCP)

0.2 μg/L MCL 3,321 <1 2 of 41

Any Various Various 3,669 4.5 29 of 41

*The proposed Alternative Maximum Contaminant Level is the primary human-health benchmark used in this circular for radon. 

†Low end of HBSL range corresponding to a 10-6 (one in a million) cancer risk. The HBSL range corresponds to a 10-6 to 10-4 cancer risk range.

Chapter 4

Sixteen contaminants—11 from geologic sources and 5 from human sources—accounted 

for nearly all (98 percent) of the instances in which concentrations were greater than 

human-health benchmarks in wells sampled in drinking-water aquifers (table 4–1). Another 

16 constituents exceeded their benchmarks in samples from only 1 or 2 wells. Some of the 

potential health effects associated with elevated concentrations of these contaminants in 

drinking water include an increased risk of cancer; various neurological, developmental, and 

reproductive effects; liver problems; and blue-baby syndrome.(16–18) Concentrations that are 

greater than human-health benchmarks are of particular concern in drinking water supplied by 

domestic wells because routine inspection or testing of these wells is not required.

Chapter 4: The Quality of Groundwater Resources For Drinking and Other Uses 37

Concentrations that are 

greater than humanhealth benchmarks are 

of particular concern in 

drinking water supplied 

by domestic wells because 

routine inspection or 

testing of these wells is not 

required.
Human-health benchmarks and other guidelines used in this assessment

What is a contaminant?

Contaminants have a wide range of sources, both manmade and geologic. Most organic chemicals in groundwater that are of 

concern for human health are manmade. In contrast, most inorganic constituents in groundwater have geologic or other natural sources, 

although their concentrations in groundwater may be altered by human activities, such as irrigation and groundwater pumping. Some 

contaminants have both manmade and natural sources. For example, nitrate in groundwater has many natural sources, but nitrate 

concentrations in groundwater underlying agricultural and urban areas commonly are higher than in other areas because of contributions from sources associated with human activities. 

But what exactly is a contaminant? The word means different things to different people. For example, a contaminant is defined 

by the Safe Drinking Water Act (SDWA) as “any physical, chemical, biological, or radiological substance or matter in water” 

(http://www.epw.senate.gov/sdwa.pdf). This broad definition of contaminant includes every substance that may be found dissolved or 

suspended in water—everything but the water molecule itself. This is not a very practical definition because this would imply that all 

water is “contaminated.” Pure water that has nothing dissolved in it does not occur naturally—not even rainfall is pure water because it 

contains, at a minimum, some dissolved gases. 

The U.S. Environmental Protection Agency defines a contaminant as “Any physical, chemical, biological, or radiological substance 

or matter that has an adverse effect on air, water, or soil.” (See http://epa.gov/region04/superfund/qfinder/glossary.html.) This definition 

is more practical and allows both manmade constituents and those with geologic sources in water to be defined as contaminants. 

However, it does not define what “adverse” means, and what may be adverse in one way might be beneficial in another. In this circular, 

a contaminant is defined as any physical, chemical, biological, or radiological substance or matter in groundwater that is manmade or 

that impairs the use of water for its intended purpose. Impairment is determined by comparing a measured concentration to benchmarks 

or guidelines. By this definition, all manmade compounds, such as pesticides and volatile organic compounds, are contaminants 

because they do not occur naturally in groundwater. If a constituent with a geologic source, such as arsenic, occurs in drinking water at 

a concentration above its human-health benchmark, it also is considered a contaminant. 

Concentrations of constituents measured for this assessment were compared to human-health benchmarks to place study 

findings in the context of human health. The benchmarks are threshold concentrations in water above which the concentration of a 

contaminant in drinking water could adversely affect human health. Human-health benchmarks were available for about two-thirds 

of the 290 constituents and properties measured for the Principal Aquifer assessments. Two types of human-health benchmarks were 

used: U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) (http://water.epa.gov/drink/contaminants/

index.cfm) and U.S. Geological Survey (USGS) Health-Based Screening Levels (HBSLs). MCLs are legally enforceable drinking-water 

standards that specify the maximum permissible level of a constituent in water that is delivered to any user of a public water system.(203)

Although MCLs are used to regulate the quality of drinking water only from public-supply sources, they also are useful for evaluating the 

quality of water from domestic and monitoring wells. An MCL was available for 53 of the constituents measured. For some constituents 

for which an MCL has not been established, the USGS, in collaboration with the USEPA and others, developed non-enforceable HBSLs 

by using standard USEPA methods for establishing drinking-water guidelines and current toxicity information(194, 195, 198) (values used 

in this report were current as of February 2012; see http://water.usgs.gov/nawqa/HBSL). An HBSL was available for 135 constituents 

measured. Radon has neither an MCL nor an HBSL, but two MCLs have been proposed.(193, 196) Copper and lead have USEPA action 

levels rather than an MCL.

In addition to human-health benchmarks, non-health-based guidelines—Secondary Maximum Contaminant Levels (SMCLs)—were 

available for some of the constituents measured in this assessment. The SMCLs are non-enforceable guidelines for concentrations 

of “nuisance” constituents in drinking water that can cause unwanted cosmetic effects such as skin or tooth discoloration; aesthetic 

effects such as unpleasant taste, odor, or color; or technical effects such as corrosion or sedimentation of plumbing or reduced 

effectiveness of water treatment.(204)

Screening-level assessments, such as this one, provide perspective on the potential relevance of detected contaminants to human 

health and can help in planning future studies(194). They are not designed to evaluate specific effects of contaminants on human health 

and are not a substitute for comprehensive risk assessments. It is important to note that occurrence of a contaminant at a concentration greater than its benchmark does not mean that adverse effects are certain to occur, because the benchmarks are conservative 

(protective) and source-water samples were collected prior to any treatment or blending that could alter contaminant concentrations in 

finished drinking water. There are water-treatment options, such as charcoal filtration, that can be used to lower the concentration of 

the contaminant to below the benchmark before the water is consumed.

38 Water Quality in Principal Aquifers of the United States, 1991–2010
Chemical use

Geochemistry

Contaminant

concentrations

in aquifers

Hydrology

Geology

Chapter 4

Because of differences in geology, hydrology, geochemistry and biogeochemistry, and 

overlying chemical use among aquifers, contaminants can exceed human-health benchmarks 

much more—or less—frequently in individual aquifers than the national statistics indicate 

(fig. 4–3; appendix 2). For example, radon concentrations exceeded the higher of the two 

proposed USEPA MCLs for radon in 25 to 50 percent of samples from wells in the crystallinerock aquifers, but in only 3.5 percent of wells sampled nationally. Concentrations of arsenic 

concentrations exceeded the MCL in more than 20 percent of wells sampled in two unconsolidated sand and gravel aquifers in the Southwest, which is nearly three times the national 

exceedance value of 6.7 percent for arsenic for all aquifers. In other aquifers, concentrations 

of arsenic, radon, and other contaminants from geologic sources did not exceed benchmarks 

in any of the sampled wells. Concentrations of dieldrin, an insecticide banned since 1987, 

exceeded its MCL in only 0.8 percent of wells nationally, but most of these occurrences were 

clustered in a few aquifers. Many of these differences and the reasons for them are investigated 

in the NAWQA Principal Aquifer assessments; findings, for those constituents that most 

frequently exceeded benchmarks, are summarized in chapters 5 and 6 of this circular.

Chapter 4: The Quality of Groundwater Resources For Drinking and Other Uses 39

Because of differences 

in geology, hydrology, 

geochemistry, and chemical 

use, contaminants can 

exceed human-health 

benchmarks much more—

or less—frequently in 

individual aquifers than the 

national statistics indicate.
circ1360_ch04_fig 03_exchhbbypabyconst_5040.ai

Percentage of wells

0 10 20 30 40

Arsenic

100 20 30 40

Uranium 

Radon 

Basin and Range carbonate

Rio Grande

High Plains

Basin and Range basin-fill

Columbia Plateau basin-fill

All aquifers

Early Mesozoic basin

Columbia Plateau basalt

Alluvial - Yellowstone

Central Valley

Mississippi River Valley Alluvial

Hawaiian volcanic

New England crystalline

Valley and Ridge crystalline

Rocky Mountain Front Range crystalline

Biscayne

Castle Hayne

Upper Floridan

Ozark Plateaus

Edwards-Trinity

Mississippian

Cambrian-Ordovician

Denver Basin sandstone

Lower Tertiary

Pennsylvanian

Piedmont and Blue Ridge crystalline

Coastal lowlands

Mississippi Embayment

Northern Atlantic Coastal Plain

Southeastern Coastal Plain

Texas coastal uplands

California Coastal Basin

Northern Rocky Mountain Intermontane

Surficial

Willamette Lowland

Alluvial - Upper Colorado

Snake River Plain basalt

Snake River Plain basin-fill

Piedmont and Blue Ridge Carbonate

Valley and Ridge carbonate

Ordovician

Woodbine

Silurian-Devonian

Glacial

50% (off scale)

47% (off scale)

0 10 20 30 40

Principal Aquifer

Radon is compared to the higher of two benchmarks; see table 4–1, p. 37.

Exceedances of human-health benchmarks by selected contaminants in 

Figure 4–3. Contaminant concentrations higher than human-health benchmarks are unequally distributed among Principal Aquifers. For example, 

the national rate, but did not exceed this benchmark at all in most of the carbonate or semiconsolidated sand aquifers. Concentrations of arsenic or 

rate, but did not exceed their MCLs at all in most of the basaltic-rock, carbonate, or sandstone aquifers.

40 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 4

EXPLANATION

Principal Aquifer rock type

Basalt and other 

volcanic rocks

Crystalline

Sandstone and carbonate

Glacial

Semiconsolidated sand

Sandstone

Carbonate

Nitrate Any pesticide 

or VOC

0 10 20 30 40 0 10 20 30 40

parts of aquifers used for drinking water

Unconsolidated sand 

and gravel

concentrations of radon exceeded the higher of two proposed MCLs 7 to 14 times more frequently in the crystalline-rock aquifers than 

uranium exceeded their MCLs two to five times more frequently in some unconsolidated sand and gravel aquifers than the national

Chapter 4: The Quality of Groundwater Resources For Drinking and Other Uses 41
Microbiological Contaminants 

In addition to chemical constituents, groundwater can contain microbiological contaminants that can be of concern for human health. Microbiological contaminants were assessed by 

measuring Escherichia coli (E. coli) and other indicator organisms in a subset (about 1,400) of 

the wells sampled in NAWQA Principal Aquifer studies. E. coli is a type of bacteria that can 

signal the presence of fecal contamination and can be associated with pathogenic microorganisms. E. coli was detected in 8 percent of the wells, indicating the potential importance of 

microbiological contaminants in untreated groundwater. E. coli and other indicator organisms 

were detected more frequently in carbonate-rock and crystalline-rock aquifers (see sidebar, 

Bacteria in groundwater in the Piedmont, Blue Ridge, and Valley and Ridge aquifers, this page) 

than in aquifers of other rock types.(19)

42 Water Quality in Principal Aquifers of the United States, 1991–2010

Photograph from the Chesapeake Bay Program

Escherichia coli

Total coliform and Escherichia coli (E. coli) bacteria are 

found in the feces of warm-blooded animals; total coliform 

bacteria are also present in soils. Total coliform bacteria 

are typically not harmful but they can signal the presence 

of other microbial contaminants. E. coli is an indicator of 

direct fecal contamination. Some strains of E. coli can cause 

gastrointestinal illness and other, serious health effects.

Bacteria in groundwater in the Piedmont, 

Blue Ridge, and Valley and Ridge aquifers

In the Piedmont, Blue Ridge, and Valley and Ridge 

aquifers, total coliform bacteria were detected in 100 percent 

of groundwater samples from springs and in 60 percent of 

samples from wells. The bacteria E. coli were detected in 

91 percent of samples from springs and 17 percent of samples 

from wells. Although detections were frequent in all aquifers 

and settings in the region, the carbonate-rock aquifers had 

higher frequencies of detection for these bacteria than 

aquifers of other rock types, as did areas overlain by agricultural land use. Karst features such as sinkholes and large 

fractures make the carbonate-rock aquifers more vulnerable 

than aquifers of other rock types to bacterial contamination. 

Groundwater in agricultural areas is more vulnerable to 

bacterial contamination because of the relatively high density 

of livestock and manure application. In undeveloped areas or 

areas of residential land use, septic systems or wildlife could 

be sources of bacteria in groundwater. Characteristics of well 

construction also are important factors that affect whether 

bacteria, particularly E. coli, are present.(127)

Human consumption of bacterial-laden water from 

domestic wells probably is common and is likely to cause a 

number of unreported illnesses.(128) Education focused on 

homeowner awareness of the benefits of maintaining well 

integrity, testing water regularly, and treating as needed could 

reduce exposure to bacteria from drinking water. Pathogenic 

viruses that cause illness—not just indicator organisms—are 

present in water drawn from wells tapping the Piedmont and 

Blue Ridge crystalline-rock aquifers and the Valley and Ridge 

carbonate-rock and siliciclastic aquifers.(129)
Chapter 4

cir1360_ch04_fig04.ai

Shallow groundwater

beneath agricultural land

Shallow groundwater

beneath urban land

0 10 20 30 40 50

Parts of aquifers used

for drinking water

Contaminants from

human sources

Exceedances of human-health benchmarks 

by one or more contaminants

Percentage of wells

24%

11%

5%

Shallow Groundwater Beneath Agricultural and 

Urban Land—A Concern for the Future?

Nitrate or manmade organic chemicals (pesticides or 

VOCs) exceeded a human-health benchmark in 24 percent of 

the shallow wells beneath agricultural areas—four times more 

frequently than in wells in the deeper parts of aquifers used for 

drinking water, underlying mixed land uses (fig. 4–4). Eleven 

percent of shallow wells beneath urban areas had a concentration of nitrate or a manmade organic chemical greater than 

a human-health benchmark. Elevated concentrations of 

contaminants from human activities are more common in 

shallow groundwater in agricultural and urban areas because 

shallow groundwater is both younger and more heavily 

influenced by chemical use at the land surface than is deeper 

groundwater. Eventually, the shallow groundwater is likely to 

move downward into the aquifer, where it may threaten the 

quality of future water supplies. Whether the contaminants 

in shallow groundwater reach the parts of an aquifer used for 

drinking-water supply depends on the physical and chemical 

processes that affect the movement of the chemicals through 

an aquifer. Movement of contaminants into deeper aquifers 

also depends on whether there are alterations to the groundwater flow system, such as groundwater withdrawals for water 

supply, that accelerate the downward movement of shallow 

groundwater (see chapters 7 and 8).

Human activities, from agriculture (top left) to urban 

and residential development (bottom right), have greatly 

altered the land surface in many places. These activities 

commonly are accompanied by waste disposal and the use 

of manmade chemicals, and can affect the quality of the 

underlying groundwater.

Photograph by Dr. Rand Schaal, Ph.D., pilot, photographer, and geologist

Figure 4–4. Contaminants from human sources—nitrate, 

pesticides, and VOCs—exceeded human-health benchmarks 

much more frequently in shallow groundwater beneath 

agricultural or urban land use than in groundwater from parts of 

aquifers used for drinking water, beneath mixed land uses.

Photograph by Lynn Betts, USDA, Natural Resources Conservation Service (NRCS)

Chapter 4: The Quality of Groundwater Resources For Drinking and Other Uses 43
Table 4–2. Iron, manganese, and a number of other constituents from geologic sources were 

outside of non-health-related U.S. Environmental Protection Agency Secondary Maximum 

Contaminant Levels.

[SMCL, U.S. Environmental Protection Agency Secondary Maximum Contaminant Level;(18) μg/L, microgram per 

liter; mg/L, milligram per liter]

Constituent SMCL Number of wells 

sampled

Percentage of 

wells outside of 

the SMCL

Number of 

Principal Aquifers 

with values outside 

of the SMCL

Any Various 3,662 56 41 of 41

Manganese 50 μg/L 3,662 23 38 of 41

Iron 300 μg/L 3,662 21 39 of 41

pH >6.5 and <8.0 3,640 18 32 of 41

Total dissolved solids 500 mg/L 3,569 18 34 of 40

Aluminum 50 to 200 μg/L 2,896 0.5 to 2.2 (6 to 13) of 37

Sulfate 250 mg/L 3,658 5.1 16 of 41

Fluoride 2 mg/L 3,655 3.8 24 of 41

Chloride 250 mg/L 3,658 3.3 25 of 41

Other Water-Quality Concerns

Nuisance Contaminants in Drinking Water

Health concerns are not the only criteria by which we 

judge the quality of our drinking water. In fact, often the most 

noticeable qualities that determine whether water is acceptable 

to consumers result from constituents that cause problems such 

as unpleasant taste or odor, staining, poor reaction with soap, 

or mineral buildup in pipes and plumbing. Iron, manganese, 

hardness, pH, total dissolved solids, and several major ions 

factor into these unwanted effects, which are a common reason 

why household water-treatment systems are used. The USEPA 

recommends limits, called Secondary Maximum Contaminant 

Levels (SMCLs), for these constituents in public water 

supplies. Overall, about half (55 percent) of the wells sampled 

in the parts of aquifers used for drinking water had levels of 

one or more unwanted constituents or properties outside of 

USEPA recommended values for drinking water (table 4–2; 

appendix 2). 

Quality of Water for Irrigation

More groundwater is pumped from the Nation’s Principal Aquifers for irrigation than for 

any other use;(4) (fig. 3–2). Quality requirements for irrigation water generally are less stringent 

than those for drinking water. However, elevated concentrations of dissolved solids—a measure 

of the salinity of the water—and several other constituents can reduce the yield of agricultural 

crops and damage soils. Concentrations of dissolved solids between 450 and 2,000 milligrams 

per liter (mg/L) in water can lead to slight to moderate restrictions on its use on crops, and 

concentrations greater than 2,000 mg/L can severely limit use.(20) Elevated concentrations 

of boron, sodium, and chloride also can lead to restrictions on the use of water for irrigation. 

Nationally, 21 percent of wells from drinking-water aquifers had concentrations of dissolved 

Iron in groundwater is not 

a health concern, but it is a 

nuisance because it can cause 

rust-colored stains on sinks, 

bathtubs, and laundry.

Photograph courtesy of Culligan

44 Water Quality in Principal Aquifers of the United States, 1991–2010
Photograph by Chris Waldron, USGS

Chapter 4

solids greater than 450 mg/L, and 1.7 percent had concentrations greater than 2,000 mg/L. 

In shallow groundwater beneath agricultural areas, 32 percent of wells had concentrations 

of dissolved solids greater than 450 mg/L, and 2.2 percent had concentrations greater than 

2,000 mg/L. Most of the wells with concentrations greater than 2,000 mg/L, which would 

severely restrict the use of the water for irrigation, were from unconsolidated or sandstone 

aquifers in the western United States or from deep, confined aquifers. More information on the 

distribution of dissolved solids is given in chapters 5 and 7.

Quality of Groundwater Flowing to Streams and Coastal Waters

Groundwater in many aquifers ultimately flows into streams, lakes, or coastal waters. 

Consequently, groundwater quality can affect aquatic life or the beneficial uses—such as 

fisheries and recreation—that we derive from these waters. Nitrogen (primarily nitrate in 

groundwater) and phosphorus are of particular concern because they can cause excessive plant 

growth, noxious algal blooms, and depleted dissolved oxygen, which are among the top impairments that degrade our streams, lakes, and estuaries.(21) Groundwater contributions to streams, 

lakes, and estuaries are not obvious and are hard to measure. Studies have shown, however, 

that groundwater discharge can provide as much as 50 percent of the flow and nitrogen load 

delivered to streams that drain to sensitive coastal waters, such as the Chesapeake Bay.(22, 23)

Under such conditions, groundwater quality is essential to consider when developing programs 

to reduce contaminant loads, such as Total Maximum Daily Loads (TMDLs), to coastal 

waters.(24) Groundwater and surface-water interactions and nitrogen in groundwater are detailed 

in the USGS circular, “Nitrogen in the Nation’s Streams and Groundwater, 1992–2004.”(25)

Excessive amounts of nutrients in streams and coastal embayments can lead to noxious algal 

blooms, such as those in an estuary along the Massachusetts coast. Nitrate from groundwater can 

be a major source of nitrogen in streams that flow to coastal embayments.

Chapter 4: The Quality of Groundwater Resources For Drinking and Other Uses 45
 

Photograph fby Bruce Lindsey, USGS

Cost of water treatment

Even when water is plentiful, it is not truly available for use unless the quality is acceptable for the intended use. Both water 

quality and availability are essential to maintaining water supply for municipal, domestic, agricultural, and recreational uses and 

for aquatic life. Where water quality is poor, there are options to reduce or remove harmful constituents, but they come with costs. 

Treatment costs and options for addressing groundwater contamination vary depending on the type of contaminant and the specifics 

of water-supply management, but treatment can cost thousands of dollars per household.

Some options available to domestic well users and public-supply providers include:

•	 installing and maintaining household treatment devices;

•	 buying drinking water, either in bottles or from another supplier;

•	 blending water (diluting a contaminated source with water from an uncontaminated source);

•	 locating and developing a new, uncontaminated source of drinking water; and

•	 building or upgrading and operating a treatment plant to remove contaminants.

A variety of point-of-use household water 

treatments are available

“My home system consists of a filter, a water softener, 

and an ultraviolet (UV) light system. I also have a reverse 

osmosis (RO) system serving the kitchen area. The filter, UV 

light, and RO systems treat for nitrate and bacteria, and cost 

about $1,600. The UV light and RO filters are changed annually 

for about $200. The water softener is for extremely hard water, 

which was recently replaced for about $1,500 and costs about 

$100 a year for salt.”

—Pennsylvania homeowner with domestic supply well

46 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 5

This chapter describes 

the sources of arsenic, 

uranium, radon, 

manganese, and 

dissolved solids and 

factors that affect their 

concentrations in the 

Principal Aquifers.

As groundwater flows, it reacts with the diverse minerals, rocks, and 

sediments that make up the aquifer and soil. Chemical constituents are 

released into the groundwater from these geologic sources. Some of 

these constituents can be a concern for human health, when present in drinking 

water, or may make the water less desirable for other uses. Differences in 

geology, recharge rates, groundwater residence times, and geochemical conditions contribute to differences in chemical concentrations among and within 

Principal Aquifers. Despite these complexities, regional patterns in the distribution of constituents from geologic sources can be recognized and, in many 

cases, understood.

Photograph by Jack Haskell

Chapter 5: Understanding Where and Why 

Constituents From Geologic Sources Occur 
Photographs by Andrew Young, Brigham Young University; 

Michael Rosen, USGS

<1 1 to 5 >5 to 10

Median arsenic concentration per study, in micrograms per liter 

Arsenic

Parts of aquifers used for drinking water

EXPLANATION

Each symbol represents a study network of 20 to 30 

wells, mostly domestic supply wells

Arsenic

Arsenic occurs naturally as a trace component in many rocks and sediments. Whether the 

arsenic is released from these geologic sources into groundwater depends on the chemical form 

of the arsenic, the geochemical conditions in the aquifer, and the biogeochemical processes that 

occur. Arsenic also can be released into groundwater as a result of human activities, such as 

mining, and from its various uses in industry, in animal feed, as a wood preservative, and as a 

pesticide. In drinking-water supplies, arsenic poses a problem because it is toxic at low levels 

and is a known carcinogen.(17, 26) In 2001, the USEPA lowered the MCL for arsenic in publicwater supplies to 10 micrograms per liter (μg/L) from 50 μg/L.(27)

Arsenic was detected in nearly half of the wells sampled in parts of aquifers used for 

drinking water (41 percent of wells, concentrations greater than 1 μg/L). Detections were more 

common and concentrations generally were higher in the western United States, especially 

in several unconsolidated sand and gravel aquifers and in a carbonate-rock aquifer, than in 

the East (figs. 5–1 and 5–2). Concentrations were greater than the MCL in 6.7 percent of all 

wells sampled in the parts of aquifers used for drinking water. In about half of the aquifers 

included in these assessments, at least one well sampled contained arsenic at a concentration 

that exceeded the MCL (fig. 5–2; table 4–1). Processes that cause arsenic to accumulate in 

groundwater are complex and differ among aquifers.

Figure 5–1. Arsenic was more frequently detected and concentrations were higher in groundwater 

in the western United States than in the East.

Arsenic occurs in minerals such 

as arsenopyrite (top) and sorbed 

to iron oxide coatings (bottom) in 

a variety of rock types.

48 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 5

Basin and Range carbonate

Rio Grande

High Plains

Basin and Range basin-fill

Columbia Plateau basin-fill

All aquifers

Early Mesozoic basin

Columbia Plateau basalt

Alluvial-Yellowstone

Central Valley

Mississippi River Valley Alluvial

Hawaiian volcanic

New England crystalline

Valley and Ridge crystalline

Rocky Mountain Front Range crystalline

Biscayne

Castle Hayne

Upper Floridan

Ozark Plateaus

Edwards-Trinity

Mississippian

Cambrian-Ordovician

Denver Basin sandstone

Lower Tertiary

Pennsylvanian

Piedmont and Blue Ridge crystalline

Coastal lowlands

Mississippi embayment

Northern Atlantic Coastal Plain

Southeastern Coastal Plain

Texas coastal uplands

Glacial

California Coastal Basin

Northern Rocky Mountain Intermontane

Surficial

Willamette Lowland

Alluvial-Upper Colorado

Snake River Plain basalt

Snake River Plain basin-fill

Piedmont and Blue Ridge carbonate

Valley and Ridge carbonate

Ordovician

Woodbine

Silurian-Devonian

86420 10 403020100 50

Arsenic in Principal Aquifers

Exceedances of the MCL, in 

percentage of wells

Median concentration by aquifer, 

in micrograms per liter

Median values in

shaded area are less than 

1 microgram per liter

Principal Aquifer rock type

EXPLANATION

All aquifers

Basalt and other volcanic rocks

Crystalline

Sandstone and carbonate

Glacial

Unconsolidated sand and gravel

Semiconsolidated sand

Sandstone

Carbonate

Figure 5–2. Principal Aquifers in the western United States that have relatively high arsenic concentrations include several unconsolidated sand and gravel aquifers, the Basin and Range carbonate aquifers, 

and the Columbia Plateau basalt aquifers. Concentrations greater than the MCL of 10 μg/L for arsenic also 

were measured in several aquifers in the East, including the early Mesozoic basin sandstone aquifers, 

glacial aquifer system, and the crystalline-rock aquifers.

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 49
Southwest Basin-Fill Aquifers—Rock Type, Climate, 

and Long Groundwater Flow Paths Yield Elevated 

Concentrations of Arsenic

Concentrations of arsenic in basin-fill aquifers of the Southwest were 

among the highest in the Nation, with concentrations greater than the 

MCL in 3 to 26 percent of wells (fig. 5–2). Geology, a dry climate, and 

long groundwater residence times are some of the reasons why arsenic 

concentrations are elevated in these aquifers.(28, 29) The Southwest basin-fill 

aquifers—the Basin and Range, California Coastal Basin, Central Valley, 

and Rio Grande aquifers or aquifer systems—are made up of unconsolidated sand and gravel sediments that originated from weathering of the 

surrounding mountains. The type of rock in the surrounding mountains is 

an important control on arsenic concentrations: concentrations are higher 

in groundwater in basin-fill sediments that are derived from volcanic or crystalline rocks than 

in groundwater in sediments that are derived from carbonate or clastic sedimentary rocks. 

Geothermal water and sulfide minerals in aquifer rocks are other geologic sources of elevated 

arsenic in the groundwater of these aquifers.(29, 30)

Residents of the Albuquerque area, New Mexico, depend on water from the Rio Grande and 

from the basin-fill aquifer system. Groundwater west and north of the city contains elevated 

concentrations of arsenic.

Johanna Siegmann Photography, http://www.johannasiegmann.com/, used with permission

50 Water Quality in Principal Aquifers of the United States, 1991–2010

circ1360 ch05_indexmap01

Basin and Range

basin-fill aquifers

California Coastal Basin aquifers

Central Valley

aquifer system

Rio Grande

aquifer system
Chapter 5

<1

1 to <5

5 to <10

≥10

Modified from Anning and others(31)

EXPLANATION

Basins with 

low recharge

Basins with

high recharge

Basin margin Middle basin

Water table

Basin lowlands

Groundwater flow path

Volcanic and

crystalline rocks

in mountains

Predicted arsenic concentrations, 

by percentage of basin area, 

in micrograms per liter

Arsenic concentrations increase as the groundwater interacts with the 

aquifer sediment along flow paths

Groundwater flows from the mountainous basin margins, where the sand and gravel 

deposits are coarse grained and thick, towards the low-lying valley floors. There is little 

recharge through the basin-fill sediments, and under natural conditions, it can take more than 

1,000 years for groundwater to reach the center of the valley. The long flow paths and slow 

groundwater flow rates result in long groundwater residence times. Thus, there is ample time 

for chemical reactions to occur that release arsenic from the sediments, and arsenic concentrations increase as groundwater moves from mountain fronts to the lowlands in the centers of 

basins (fig. 5–3). In basins where recharge rates are relatively high (greater than about 2 inches 

per year—still quite low compared with humid regions), less arsenic accumulates than in 

the more arid areas. In general, the more water that flows through the system, the lower the 

concentrations of constituents, such as arsenic, from geologic sources are likely to be. Basins 

that have no natural surface-water outflow—water leaves these closed basins only through 

evapotranspiration or deep groundwater underflow—are particularly vulnerable to the accumulation of arsenic and other constituents from geologic sources.(15, 28, 30)

Figure 5–3. In the Southwest basin-fill aquifers, arsenic concentrations increase along flow paths 

from recharge areas at mountain fronts to low-lying areas in the valley center. Where recharge rates 

are higher and more water moves through the system, concentrations still increase along flow paths 

but are lower throughout the basin than in more arid areas.

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 51
NV

OR

ID

CA

AZ NM

UT CO

Pacif ic

Ocean

San Diego

San Francisco

Predicted arsenic concentrations 

in the Southwest basin-fill 

aquifers, in micrograms per liter

<1.0

1.0 to <5.0

5.0 to <10.0

≥10.0

Concentrations not predicted 

in this area

Los Angeles

Phoenix

Sacramento

Tucson

Salt Lake City 

Las Vegas

Albuquerque

Reno

Modified from Anning and others(31)

Predicted arsenic concentrations in the Southwest basin-fill aquifers

N

 EXPLANATION

Southwest

Principal Aquifers

Figure 5–4. Predicted concentrations of arsenic in groundwater exceed the MCL of 10 μg/L in 43 percent of the areal extent of the 

four Southwest basin-fill aquifers. The Basin and Range basin-fill aquifers in Nevada, Utah, southern California, and western Arizona 

encompass the largest area where arsenic concentrations are predicted to exceed the MCL, largely due to the presence of volcanic 

rocks, the arid to semiarid climate, and many closed basins (basins with no surface-water outflow).

A regional study predicted that arsenic concentrations would exceed the MCL in groundwater beneath about 43 percent of the areal extent of the four Southwest basin-fill aquifers(31)

(fig. 5–4). This percentage is higher than the percentage of measured concentrations from wells 

because the areal extent of the basin-fill aquifers includes large areas with arsenic-bearing 

rocks and constricted groundwater flow—many of these areas are sparsely populated, with little 

groundwater pumping and few wells. The predicted concentrations can be used to anticipate 

the water-quality conditions likely to be encountered in unsampled or undeveloped areas and 

to understand the limitations that water quality can impose on water-supply development—for 

example, by adding costs for water treatment (see sidebar, The cost of arsenic contamination, 

p. 53). Such predicted concentrations also can be used to represent concentrations of contaminants in water consumed in the past in areas where there are no monitoring data.

52 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 5

Photograph by Jeff Ross

The arsenic treatment system for Fallon, Nevada, adds dissolved iron to the water. The iron reacts with the 

dissolved arsenic to form particles that are then filtered out of the water.

The USEPA estimated in 2001 that the annual cost to reduce 

arsenic concentrations to below the MCL would range from 

$0.86 to $32 per household for customers of large public water 

systems (more than 10,000 people) to $165 to $327 per household 

for very small systems (25–500 people).(27) Water supplies in 

the southwest are limited and often naturally contain arsenic 

concentrations high enough to require treatment. According 

to fiscal year 2010 statistics,(130) 274 public water systems in 

California, Nevada, Utah, Arizona, and New Mexico—mostly 

The cost of arsenic contamination

small systems that lack a large customer base to pay for water 

treatment—had a water source containing arsenic concentrations that exceeded the MCL for arsenic. In Fallon, Nevada, 

groundwater supplying about 8,400 residents and a nearby naval 

air station regularly contains arsenic concentrations exceeding 

the MCL. In 2004, a treatment facility was installed for arsenic 

removal at a cost of $19 million.(131) Upgrades to water systems 

throughout Nevada to remove arsenic are estimated to cost tens 

of millions of dollars.(97)

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 53
Photograph by Kelly Warner, USGS

Arsenic concentration, in 

percentage of wells

> 10 μg/L

≥ 1 and ≤ 10 μg/L

< 1 μg/L

pH

Acidic

(pH < 7)

Alkaline

(pH ≥ 7)

Anoxic Oxic

Redox

EXPLANATION

Arsenic in the Glacial Aquifer System—The Important 

Roles of Redox and pH

Arsenic concentrations in the glacial aquifer system were higher in 

deeper aquifers, which are used for drinking water, than in shallow parts of 

the aquifer system: 12 percent of wells from the deeper aquifers used for 

drinking water had arsenic concentrations that exceeded the MCL, whereas 

only 4 percent of shallow wells had concentrations that exceeded the MCL. 

This difference is largely because groundwater in the deeper aquifers is 

more likely to be anoxic, and redox conditions, along with pH, are an 

important control on concentrations of arsenic in these aquifers. 

Much of the arsenic in the glacial aquifer system is sorbed to iron 

and manganese oxides that coat the aquifer sediments.(8, 32) In anoxic 

groundwater, arsenic occurs in its reduced redox state, arsenite (AsIII), 

which is not strongly attracted to iron oxides and largely remains dissolved in water. Moreover, 

iron oxides themselves can dissolve under anoxic conditions, eliminating that mechanism of 

retaining arsenic on aquifer solids. In contrast, under oxic conditions, arsenic occurs in its 

oxidized state of arsenate (AsV), which is strongly attracted to iron oxide minerals and will 

sorb onto these surfaces, leading to lower concentrations of arsenic in groundwater. Sorption of 

arsenate is pH dependent, however, and does not occur much in alkaline groundwater. Consequently, arsenic concentrations were higher in anoxic groundwater than in oxic groundwater 

and higher in alkaline groundwater than in acidic groundwater throughout the glacial aquifer 

system (fig. 5–5). Concentrations were highest in the central region of the glacial aquifer 

system, where strongly reducing conditions are present in deep groundwater in buried bedrockvalley aquifers (fig. 5–6).

Figure 5–5. In the glacial aquifer system, arsenic concentrations 

were higher in anoxic groundwater and in alkaline groundwater 

than in oxic or acidic groundwater. Arsenic that is sorbed to iron 

and manganese oxides under oxic conditions is released into 

groundwater by desorption or by the dissolution of the oxide 

minerals under reducing conditions; pH also is important in 

this process.

The sands, gravels, and fine-grained sediments of the glacial 

aquifer system extend across the northern United States in river 

valleys, buried bedrock valleys, and broad lenses. These aquifers 

are important water sources, providing more water for public 

supply than any other aquifer system in the United States.

54 Water Quality in Principal Aquifers of the United States, 1991–2010

circ1360 ch05_indexmap02

Glacial 

aquifer system
Chapter 5

Photographs by Denis Leblanc, USGS; Joseph Ayotte, USGS 

East

Central West central

 West

 West

EXPLANATION

Glacial aquifer system

≤ 1

Arsenic concentration in wells, in micrograms per liter

>1 to 10 >10

Redox conditions in the 

glacial aquifer system can 

be apparent from the color of 

the sediments. Sand grains 

from parts of the aquifer 

that are anoxic (top) are 

grayish in color and lack 

the iron-oxide coatings that 

gives sand from oxic parts of 

the aquifer (bottom) its rusty 

reddish hue.

Figure 5–6. Arsenic concentrations were relatively high in the glacial aquifer system, especially 

in wells sampled in the deeper parts of the aquifer system used for drinking water. Wells in deeper 

parts of the aquifer in the western and central regions of the glacial aquifer system more frequently 

contained arsenic at concentrations that exceeded the MCL of 10 μg/L than did wells in the westcentral and eastern regions.

Anoxic conditions are more common in the glacial aquifer system than in many other 

Principal Aquifers because of the presence of buried organic carbon and reduced minerals such 

as pyrite (iron sulfide) in the glacial sediments.(15) However, redox and pH also are important 

controls on arsenic concentrations in a number of other Principal Aquifers. In the Mississippi 

River Valley alluvial aquifer, arsenic concentrations exceeded the MCL in 9 percent of wells 

(fig. 5–2). These elevated arsenic concentrations were present only when concentrations of 

dissolved iron also were high, suggesting that, as in the glacial aquifer system, dissolution of 

iron oxides was a likely cause of the high arsenic.(33) In the New England crystalline aquifers, 

the importance of pH-dependent desorption from iron oxides is indicated by the more frequent 

occurrence of elevated arsenic concentrations in alkaline groundwater than in acidic groundwater, although multiple sources and mechanisms for arsenic in groundwater are likely in these 

aquifers.(34, 35)

Although anoxic conditions generally are more common in older, deeper groundwater 

than in young, shallow groundwater, redox conditions can be quite variable across short 

distances. Consequently, concentrations of arsenic in groundwater, when they are strongly 

influenced by redox conditions, are difficult to predict precisely. Water testing for arsenic is 

essential in this type of hydrogeologic setting to identify drinking-water supplies that contain 

concentrations of potential health concern.

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 55
Figure 5–7. Only two rock 

types in the Piedmont and Blue 

Ridge crystalline-rock aquifers 

and the early Mesozoic basin 

aquifers had concentrations of 

arsenic greater than the MCL 

of 10 μg/L. Nine percent of 

samples from wells screened 

in either of these rock types 

had arsenic concentrations 

greater than the MCL, but no 

concentrations greater than 

the MCL were measured in 

samples from wells in the other 

rock types in these aquifers 

or in the adjacent Valley and 

Ridge aquifers.

Arsenic in the Piedmont and Blue Ridge Crystalline, 

New England Crystalline, and Early Mesozoic Basin 

Aquifers—Local Patterns of Occurrence

Geology plays a key role in determining where arsenic occurs in 

groundwater, and in some aquifers, elevated concentrations are found 

only in specific rock types. The Piedmont and Blue Ridge crystalline, 

New England crystalline, and early Mesozoic basin aquifers each 

contain several different types of fractured igneous, metamorphic, and 

sedimentary rocks. These aquifers are used for public supply in some 

areas, but they are especially important sources of domestic supply 

in rural areas, where they can be the only source of water supply. In 

the Piedmont and Blue Ridge crystalline aquifers, concentrations of 

arsenic greater than the MCL were measured only in wells from one 

type of rock, metamorphosed sedimentary clastic rocks (fig. 5–7)(36). Similarly, in the early 

Mesozoic basin aquifers, only wells in the sedimentary rock composed of ancient lake sediments had concentrations of arsenic greater than the MCL. Within each of these rock types, 

9 percent of wells had concentrations greater than the arsenic MCL. Patterns of occurrence 

among rock types also were apparent in parts of the New England crystalline-rock aquifers. 

Arsenic concentrations were greater than the MCL in 11 to 25 percent of wells sampled in 

areas of certain calcareous metasedimentary rocks and granites, whereas concentrations 

exceeded the MCL in only 3 to 5 percent of wells in another type of granite and in several 

other metamorphic rock types.(35, 37, 38) Local patterns such as these can be used to evaluate 

how likely it is that supply wells in unsampled areas have elevated concentrations of arsenic 

and to guide testing regulations for domestic wells in rural areas of domestic supply.

EXPLANATION

Study-area boundary

Aquifers where arsenic was 

 detected in at least one 

 sample at a concentration 

 greater than 10 micrograms 

 per liter 

≤ 1

>1 to 10

>10

Arsenic concentration, in 

 micrograms per liter

PA

WV

KY

TN

GA

NJ

MD

VA

AL

NC

SC

NY

DE

 Modified from Lindsey and others(36)

56 Water Quality in Principal Aquifers of the United States, 1991–2010

Piedmont and

Blue Ridge

crystalline

aquifers

Early 

Mesozoic

basin

aquifers

New England

crystalline aquifers
Chapter 5

Granite rocks, such as those shown on Pikes 

Peak in the Rocky Mountains of Colorado, are a 

source of uranium to groundwater.

Photograph by Steve Krull, copyright istockphoto.com

Photograph from alptran, copyright istockphoto.com

Radionuclides

Many people might be surprised to learn that drinking-water sources can contain 

radioactive elements (radionuclides). Radionuclides in groundwater are primarily from 

geologic sources and include isotopes of uranium, radon, radium, polonium, and lead (see 

sidebar, What are radionuclides and which ones are important in groundwater?, p. 63). Rock 

type, groundwater geochemistry, and, in some cases, human modifications to flow systems 

influence the distribution of radionuclides in groundwater. The radioactive decay process 

itself adds complexity because radionuclides transform into different elements. Radionuclides 

in drinking-water sources can be a concern for human health because several are toxic or 

carcinogenic.

Uranium and Radon

Uranium is a common trace element in many rock types, but it is particularly enriched in 

crystalline rocks, such as granites, and in sediments derived from crystalline rocks. Because 

uranium is highly soluble in its oxidized forms but only slightly soluble in its reduced forms, its 

mobility in groundwater depends on redox conditions. Uranium sorption is pH dependent, and 

uranium can form bonds with other ions that keep it in solution over a wider range of conditions than otherwise possible. Although weakly carcinogenic, uranium is chemically toxic, and 

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 57
is a concern for human health because it causes kidney damage at elevated concentrations when 

consumed in drinking water.(39)

Uranium was detected (concentrations greater than 1 μg/L) in 35 percent of wells sampled 

in the parts of aquifers used for drinking water but exceeded the USEPA MCL of 30 μg/L in only 

1.6 percent of samples nationally. Concentrations in groundwater were higher in the western 

United States than in the East (fig. 5–9). Low recharge rates, oxic groundwater, long flow paths, 

and high concentrations of naturally occurring uranium in rocks and soils contribute to the 

accumulation of uranium in the unconsolidated sand and gravel aquifers of the West. Crystallinerock aquifers in the Rocky Mountains and in the northeastern United States also had relatively 

high concentrations of uranium, as did several sandstone aquifers in the mid-Atlantic region and 

in Colorado (fig. 5–8). In these aquifers, the MCL was exceeded in 3 to 26 percent of wells.

Figure 5–8. Unconsolidated sand and gravel aquifers and one crystalline-rock aquifer, all in the West, had the highest concentrations 

of uranium among Principal Aquifers. Nearly all the exceedances of the MCL for uranium of 30 μg/L were in these aquifers. 

Concentrations of radon, on the other hand, were higher in the three crystalline-rock aquifers than in any other aquifer. The higher of 

two proposed MCLs for radon (4,000 pCi/L) was exceeded most frequently in groundwater samples from the crystalline-rock aquifers.

Basin and Range carbonate

Rio Grande

High Plains

Basin and Range basin-fill

Columbia Plateau basin-fill

All aquifers

Early Mesozoic basin

Columbia Plateau basalt

Alluvial-Yellowstone

Central Valley

Mississippi River Valley Alluvial

Hawaiian volcanic

New England crystalline

Valley and Ridge crystalline Rocky Mountain Front Range crystalline

Biscayne

Castle Hayne

Upper Floridan

Ozark Plateaus

Edwards-Trinity

Mississippian

Cambrian-Ordovician

Denver Basin sandstone

Lower Tertiary

Pennsylvanian

Piedmont and Blue Ridge crystalline

Coastal lowlands

Mississippi embayment

Northern Atlantic Coastal Plain

Southeastern Coastal Plain

Texas coastal uplands

Glacial

California Coastal Basin

Northern Rocky Mountain Intermontane

Surficial

Willamette lowland

Alluvial-Upper Colorado

Snake River Plain basalt

Snake River Plain basin-fill

Piedmont and Blue Ridge carbonate

Valley and Ridge carbonate

Ordovician

Woodbine

Silurian-Devonian

403020100 50

Exceedances of the MCL, 

in percentage of wells

Median concentration by 

aquifer, in micrograms per liter

1086420 12 0

Median concentration by 

aquifer, in picocuries per liter

0 1,000 2,000 3,000 4,000 80604020 100

Exceedances of the MCL, 

in percentage of wells

Median values in 

shaded area are less 

than 1 microgram per liter

Uranium in Principal Aquifers Radon in Principal Aquifers

EXPLANATION

Basalt and other volcanic rocks

Crystalline

Sandstone and carbonate

Glacial

Unconsolidated sand and gravel

Semiconsolidated sand

Sandstone

Carbonate

Concentrations greater than

proposed MCLs

4,000 pCi/L

300 pCi/L

All aquifers Principal Aquifer rock type

58 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 5

Each symbol represents a study network of 20 to 30 wells, 

mostly domestic supply wells

Uranium in groundwater

Parts of aquifers used for drinking water

<1 1 to 3 >3 to 11

Median uranium concentration per study, in micrograms per liter

EXPLANATION

Radon

Parts of aquifers used for drinking water

Each symbol represents a study network of 20 to 30 wells, 

mostly domestic supply wells

<300 300 to 1,000 >1,000 to 2,150

Median radon concentration per study, in picocuries per liter

EXPLANATION

Data from Phillips and others(202)

Uranium (238-U) in soil, from aerial gamma-ray surveys, 

in parts per million equivalent uranium (ppm eU)

Uranium in soil

EXPLANATION

High 5.4 

Low -0.44

Figure 5–9. Concentrations of uranium in groundwater (top 

left) generally were higher in the West and Northeast than in 

other parts of the United States. The concentration of uranium 

in aquifer rocks, which is reflected in concentrations in 

soils (bottom left), is one factor that influences groundwater 

concentrations. High concentrations of radon in groundwater, 

in contrast, (top right) occurred primarily in the Northeast.

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 59
circ1360 ch05 custom sidebar2_fig 01

Principal Aquifer rock type

EXPLANATION

 Basalt and other volcanics

 Sandstone and carbonate

 Unconsolidated sand and gravel

 Crystalline

 Carbonate

 Sandstone

 Semiconsolidated sand

 Glacial

180

150

120

90

2,000

0

Radon, in picocuries per liter

30

60

0 4,000 6,000 8,000 10,000

Uranium, in micrograms per liter

High uranium 

and radon

High

uranium

High

radon

U.S. Environmental Protection 

Agency MCL or proposed AMCL

Uranium-238 and radon-222 have the same geologic source 

because radon-222 is a daughter isotope in the uranium-238 decay 

series. Why, then, don’t high concentrations of uranium and radon 

occur together in groundwater? The answer lies in the geochemical 

and radiological differences between the two elements. Uranium 

is more likely to stay dissolved when groundwater is oxic, pH is 

slightly alkaline, and there are high concentrations of bicarbonate 

or sulfate. Radon, on the other hand, is less affected by groundwater geochemistry and is dissolved in groundwater over a wide 

range of pH and redox conditions. When uranium is present in 

aquifer materials but geochemical conditions do not favor its 

mobility, concentrations of radon in groundwater can be high when 

concentrations of uranium are not. The difference in their radioactive decay rates is another factor. Uranium-238 has a half-life in 

the billions of years, whereas radon-222 has a half-life of less than 

4 days. This difference in half-life means that uranium-238 can 

travel long distances from its source in aquifer rocks or sediments 

before it decays. Radon-222, on the other hand, can travel only 

a short distance from its source before it decays to its daughter 

products. Finally, radon-222, a dissolved gas, can escape from the 

water, whereas uranium, a dissolved ion, cannot.

Uranium and Radon—Why don’t they occur together in groundwater?

High concentrations of uranium and radon don’t 

often co-occur.

60 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 5

Uranium in groundwater is affected by geochemical 

conditions, whereas radon is not. A, Uranium occurred 

primarily in oxic conditions only, whereas radon occurred 

in oxic and anoxic redox conditions. B, High concentrations 

of uranium were measured mostly in groundwater with a 

pH range of 6.5 to 8, whereas high concentrations of radon 

were measured in groundwater with a wider pH range 

of 5 to 9. C, Uranium concentrations were relatively high 

primarily in water with high concentrations of bicarbonate, 

whereas radon occurred in water regardless of the 

concentration of bicarbonate.

EXPLANATION

25th percentile

10th percentile

75th percentile

50th percentile

 (median)

90th percentile

Outlier

Outlier

50 percent of data

fall within the box 

80 percent of data fall 

 between endpoints of whiskers 

Boxplots

Boxplots are used to illustrate how results are distributed within a group. The 

“box” ranges from the 25th to the 75th percentile and represents 50 percent of the 

data. The horizontal line in the middle of the box is the median value—one-half of the 

values in the group are greater than the median and one-half are less. 

Percentiles describe the percentage of values in a group that are less than the 

given value: 25 percent of the values in a group are less than the 25th percentile; 

75 percent of the values in a group are less than the 75th percentile. The median is also 

the 50th percentile.

If, for example, the 75th percentile for the measured concentration of a contaminant in a group of wells is equal to the human-health benchmark for that contaminant, 

then 75 percent, or three-fourths, of the wells have a concentration of that contaminant 

less than the benchmark, and 25 percent, or one-fourth, have a concentration greater 

than the benchmark.

The “whiskers” (vertical lines) in these figures extend to the 10th and 90th 

percentiles; box and whiskers together represent 80 percent of the data. Values 

greater than the 90th or less than the 10th percentile are shown as individual 

points (outliers). Outliers are not shown on all graphs. Uranium, in micrograms per liter

14

12

10

8

0

4

6

2

0

1,000

4,000

3,000

2,000

Oxic Anoxic Oxic Anoxic

Radon, in picocuries per liter

180

150

120

90

0

30

60

2,000

0

4,000

10,000

8,000

6,000

Uranium, in micrograms per liter

2 3 4 5 6 7 8 10 11 9

0 200 400 600 800 1,000 1,200

Bicarbonate, in milligrams per liter

Radon, in picocuries per liter

pH, in standard units

180

150

120

90

0

30

60

Uranium, in micrograms per liter

Radon, in picocuries per liter

2,000

0

4,000

10,000

8,000

6,000

A. Redox conditions B. pH

C. Bicarbonate

Uranium Radon

Radon

EXPLANATION

Radon

EXPLANATION

Uranium

Uranium

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 61
Modified from Otton and others(179)

Figure 5–10. Radon gas (red dots) that is dissolved in 

groundwater can be pumped from a well and travel through a 

household water distribution system. Once the water is aerated 

through a faucet, the radon is released into the air and can 

be inhaled when people are close to showerheads or drinking 

directly from faucets.

Radon (radon-222) is present in most groundwater in the 

United States and was detected in 94 percent of wells sampled 

in the parts of aquifers used for drinking water. Radon in water 

is a dissolved gas that does not react with other chemicals. 

When water that contains radon is used in a home, most of 

the radon is released from the water into the air and can be 

inhaled (fig. 5–10). Inhalation of radon poses a risk of lung 

cancer.(40) The USEPA has proposed an MCL of 300 pCi/L 

and an Alternative MCL (AMCL) of 4,000 pCi/L for radon 

in public water systems.(40–42) The lower proposed MCL for 

radon would apply to States and public water systems that 

do not develop programs to address health risks from radon 

in indoor air; the higher proposed AMCL would apply to 

States and public water systems that have established such 

programs. Concentrations exceeded the lower proposed MCL 

in 64 percent of wells, including at least one well from every 

Principal Aquifer in this study. The proposed AMCL was 

exceeded in only 3.6 percent of wells. Most of the concentrations greater than 4,000 pCi/L were measured in crystallinerock aquifers in the Northeast, the mid-Atlantic region, and 

Colorado (figs. 5–8 and 5–9).

62 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 5

Natural radioactive decay series for parent elements uranium-238 and thorium-232. The radionuclides produced in these decay 

series include those that occur in groundwater and can pose health concerns in drinking-water sources (purple, red, and green 

circles). Polonium-210 (dark blue) and lead-210 (light blue) can also occur in groundwater under certain conditions and be health 

concerns, but few data are available on their concentrations in groundwater. Lead-206 and lead-208 (dark grey) are stable 

elements at the end of the decay series.

A radionuclide is an atom (element) with an unstable nucleus 

(core). The nucleus of the atom has excess energy that is released 

by different types of radioactive decay. Radionuclides in rocks and 

soils are produced naturally by the decay of radioactive parent 

elements such as uranium and thorium.

When an atom undergoes radioactive decay, it can become 

a different element, or it can become a different isotope of the 

same element. Isotopes are atoms of an element that contain the 

same number of protons (the same atomic number) but a different 

numbers of neutrons. For example, uranium-234, uranium-235, 

and uranium-238 each have 92 protons—which defines them as 

uranium—but the number of neutrons differs; the number after 

the element name refers to the number of protons plus neutrons 

in the atom. The most common mechanisms for radioactive decay 

are emission of alpha particles (loss of two protons and two 

neutrons) and emission of beta particles (loss of an electron). 

Through radioactive decay, parent isotopes produce intermediate 

radioactive daughter isotopes with predictable half-lives—the 

time needed for half of the initial amount of a radionuclide to 

decay. The half-lives of isotopes vary from fractions of a second 

to billions of years.

Radioactive isotopes in water typically are measured by 

the amount of radioactive energy that is released from their 

decay, called activity. There are several different units used for 

measuring the activities of isotopes; picocuries per liter of water 

is the most commonly used unit in the United States. Uranium is 

an exception; it commonly is measured in water by its mass using 

micrograms per liter, which is a concentration rather than an 

activity. Nearly all the natural uranium in water is uranium-238, 

and activity and concentration units are consistently comparable 

for uranium. In this study, radon and radium were measured as 

activities and uranium was measured as a concentration, which 

is consistent with common practice and with the MCLs or other 

human-health benchmarks available for these constituents. For 

simplicity, however, both units of measurements are referred to 

as concentrations in this report.

What are radionuclides and which ones are important in groundwater?

Modified from Zapecza and Szabo(132)

U-238

4.5 billion

years

U-234

24 

days

Th-230

75,000

years 

Pa-234

1.2 

minutes

Th-234

24 

days

Rn-222

3.8

days Po-214

14 milliseconds

Pb-214 27 minutes

Bi-214

20 

minutes

Bi-210 5

days

Pb-210 22

days

Ra-226

1,600

years

Po-218

3.1

minutes

Ra-228

5.8

years

Th-232

14 billion

years

Ac-228

6.1 

hours

Th-228

1.9

years

Rn-220

55 seconds

Po-216

0.16

seconds

Pb-212

11

hours

Ra-224

3.6

days

Bi-212

61

minutes

Po-212

0.3 microsecond

Pb-208

Stable

Tl-208

3.1

minutes

Po-210

140

days

Pb-206

Stable

Bismuth

Uranium

Protactinium

Thorium

Radium

Radon

Polonium

Lead

Bismuth

Thorium

Actinium

Radium

Radon

Polonium

Lead

Thallium

Uranium-238 Thorium-232

Radioactive decay series

Alpha decay

Beta decay

U-234

24 

days Half-life

Isotope name

EXPLANATION

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 63
< 1 1 to 5 > 5 

Radium concentration in sampled wells, in picocuries per liter

Modified from Szabo and others(43)

EXPLANATION

Radium

Radium

Radium can enter groundwater by dissolution of aquifer materials, by desorption from 

rock or sediment surfaces, and by release from minerals during radioactive decay. Radium-226 

and radium-228 are the two most common isotopes of radium, and both are carcinogenic. 

Radium dissolved in drinking water is a human-health concern because it accumulates in bone 

and other tissues, thereby increasing lifetime cancer risks. The USEPA MCL for radium is 

5 pCi/L for the combined concentration of radium-226 and radium-228.

Nationally, 3.2 percent of 1,270 wells for which samples were analyzed for radium-228 

and (or) radium-226 had concentrations greater than the MCL for combined radium.(43, 44)

Elevated concentrations were more common in groundwater in the eastern and central United 

States than in other regions (fig. 5–11). 

Figure 5–11. Nationally, about 3 percent of sampled wells had concentrations of radium greater 

than the MCL of 5 pCi/L. Nearly all these elevated concentrations were in the eastern and central United States. Radium data are shown here for individual wells, rather than grouped by 

groundwater study.

64 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 5

Cambrian-Ordovician and Ozark Plateaus

0.1

Radium-226 plus radium-228, in picocuries per liter

1 10 50

 Northern Atlantic Coastal Plain

Floridan and Surficial

New England crystalline

Coastal Plain aquifers in the south-central 

United States 

Early Mesozoic basin 

Glacial

Denver Basin and Lower Tertiary

High Plains

Southeastern Coastal Plain

Valley and Ridge and Mississippian

Alluvial and small-basin aquifers in the West

Rio Grande and Basin and Range basin-fill

Central Valley and Willamette Lowland

Columbia Plateau and Snake River Plain basalt

and Hawaiian volcanics

Principal Aquifer

MCL

Modified from Szabo and others(44)

See sidebar, Boxplots, p. 61

Figure 5–12. Concentrations of radium were higher than the MCL of 5 pCi/L in some wells in 

a number of aquifers, including the Cambrian-Ordovician and Ozark Plateau aquifers and the 

Northern Atlantic Coastal Plain aquifer system. Geochemical conditions rather than abundance 

of the parent elements uranium and thorium in aquifer materials were primarily responsible for 

these elevated concentrations.

Aquifers with the highest combined radium concentrations were the CambrianOrdovician aquifers and Ozark Plateau aquifers (Iowa, Kansas, Missouri, Oklahoma, and 

Arkansas) and the Northern Atlantic Coastal Plain aquifer system (New Jersey, Delaware, 

Maryland, Virginia, and North Carolina). More than 20 percent of groundwater samples from 

these aquifers had concentrations of radium greater than the MCL (fig. 5–12). Concentrations 

of radium are high in these aquifers because geochemical conditions favor radium mobility 

rather than because the aquifer materials are enriched in uranium or thorium. Anoxic conditions, low pH, and high concentrations of dissolved solids (especially cations such as calcium, 

barium, and magnesium) hinder the sorption of radium onto aquifer sediments and thus favor 

the release of radium into the surrounding groundwater. The factors that enhance radium 

mobility also are quite different from those that favor the mobility of uranium. Although one 

of the common isotopes of radium (radium-226) is derived from uranium, high concentrations 

of uranium and radium rarely coincided in the Principal Aquifers studied.

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 65
Redox 

conditions 

0 4020 60 80

Uranium in groundwater

Alluvial

Aquifer

Dawson

Denver

Arapahoe

LaramieFox Hills

MCL

All below detection

900 1,000

Concentration, in micrograms per liter

Dawson aquifer

Water table

Underlying confining unit

Arapahoe aquifer

Denver aquifer

Laramie-Fox Hills aquifer

Unsaturated zone Stream

Stream

Mountain

front

Alluvial aquifer Alluvial aquifer

Denver Basin aquifer system

Anoxic Oxic

Mixed

Redox conditions, in

percentage of wells

EXPLANATION

See sidebar, Boxplots, p. 61

Denver Basin Aquifer System—Redox Conditions and 

Uranium Concentrations Vary With Depth and May Be 

Changed by Human Activities

Layered sandstones make up the Denver Basin aquifer system, east of the 

Rocky Mountains in Colorado. Concentrations of uranium in drinking-water 

wells from deep parts of the aquifer system are low. However, 19 percent of 

samples of shallow groundwater from the shallow Dawson sandstone and 

overlying alluvial aquifer had concentrations of uranium greater than the 

MCL (fig. 5–13). This difference is because there is more dissolved oxygen 

in groundwater at shallow depths than deep in the aquifer system. Uranium, 

a trace component of the layered sandstones and sediments, is more soluble 

in oxygenated groundwater than in groundwater in which dissolved oxygen 

has been depleted. The decrease in dissolved oxygen with depth is natural; oxygen from the 

atmosphere is depleted by reactions with organic matter and reduced minerals as groundwater 

recharge moves further and deeper into the aquifer. Groundwater recharge from precipitation is 

very low, generally less than 1 inch per year. Irrigation and pumping for water supply, however, 

have increased the flow of oxygenated water into the aquifer and increased the rate at which 

groundwater moves downward.(7) These changes have the potential to flush uranium and other 

constituents from the shallow aquifers into the deeper layers used for drinking water. More 

information and examples of how changes like these can alter groundwater quality are provided 

in chapter 7.

Figure 5–13. Uranium 

concentrations are much 

higher in shallow layers of 

the Denver Basin aquifer 

system than in deeper layers 

because oxic groundwater 

(water with dissolved oxygen) 

at shallow depths keeps the 

uranium dissolved. Nineteen 

percent of groundwater 

samples from the shallow 

alluvial and Dawson sandstone 

aquifers had uranium 

concentrations greater than 

the MCL of 30 μg/L, whereas 

no concentrations greater 

than the MCL were measured 

in samples from the deeper 

sandstone aquifers.

66 Water Quality in Principal Aquifers of the United States, 1991–2010

circ1360 ch05_indexmap04

Denver Basin

aquifer system
Chapter 5

Northern Atlantic Coastal Plain Surficial Aquifer System—

Naturally Acidic Waters and Elevated Radium

Weathered, quartz-rich sediments in the Northern Atlantic Coastal Plain 

surficial aquifer system have little capacity to buffer the pH of naturally acidic 

rainfall and soil water. Consequently, groundwater in this aquifer system 

typically is acidic; the median pH of groundwater from wells in aquifer 

studies was about 5. These conditions are optimal for dissolving or desorbing 

radium from aquifer sediments and contribute to the elevated concentrations 

of radium(43–45) in parts of the aquifer system (fig. 5–14).

Groundwater concentrations of radium also were greater in agricultural 

areas than in nonagricultural areas(45, 46) in parts of the Northern Atlantic 

Coastal Plain aquifer system (see chapter 7). The influx of nitrogen from 

fertilizer and septic-system effluent can enhance the natural acidity of groundwater in the 

aquifer because geochemical reactions that transform organic nitrogen and ammonia to nitrate 

(including nitrification; see chapter 3) also generate hydrogen ions (acidity). Lime commonly 

is applied where nitrogen fertilizers are used, and resulting ion-exchange reactions involving 

ions in the lime also can stimulate the release of radium from aquifer sediments into the 

groundwater. Radium concentrations decrease with depth in these areas as acidic groundwater 

is slowly neutralized (the pH is increased) by geochemical reactions with aquifer sediments. 

The presence of radium at concentrations greater than the MCL has led to new guidelines 

for the testing of domestic wells in parts of Maryland and New Jersey.(47, 48) Because radium 

generates most of the gross alpha radiation (see sidebar, What are radionuclides and which 

ones are important in groundwater?, p. 63) in acidic groundwater of the Northern Atlantic 

Coastal Plain aquifer system, testing for gross alpha radiation, which is relatively quick and 

inexpensive, is used to identify groundwater that is likely to contain elevated concentrations 

of radium.(49) Radium can be remediated (lowered or removed) in drinking water with a 

well-maintained water softener (cation-exchange) system(44, 50) or by many other options.

Figure 5–14. Concentrations 

of radium exceeded the MCL of 

5 pCi/L in the Northern Atlantic 

Coastal Plain aquifer system 

in areas with well-weathered 

quartz-rich sediments and 

naturally acidic groundwater 

and in areas of agricultural 

and urban land use. In 

agricultural and urban areas, 

the groundwater can become 

more acidic as the result of 

biogeochemical reactions 

involving applied nitrogen 

compounds.(49)

circ1353ch06 rad fig7

Chesapeake Bay

NEW 

JERSEY

MARYLAND

VIRGINIA

DELAWARE

ATLANTIC OCEAN

Delaware Bay

PENNSYLVANIA

N

< 2

≥ 2 and < 5

≥ 5

Land cover

Urban

Agricultural

Forested

Wetland

Water

Radium, picocuries

 per liter

EXPLANATION

Modified from Denver and others(45)

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 67

circ1360 ch05_indexmap05

Northern Atlantic 

Coastal Plain 

aquifer system
Photograph By D.R. Leblanc, USGS

Anoxic Oxic

Manganese in anoxic and oxic groundwater

Manganese concentration in 

percentage of wells

≥ 1 to 30 μg/L 

> 30 to 300 μg/L < 1 μg/L

> 300 μg/L 

EXPLANATION

Manganese dissolved in anoxic, sewage-contaminated 

groundwater flows to the shore of this pond on Cape Cod, 

Massachusetts. When the manganese comes into contact 

with the oxygen in the atmosphere, it oxidizes and precipitates, 

forming the dark colored manganese oxide coatings on 

cobbles along the shoreline.

Manganese 

Manganese is a nuisance in water supplies because it stains plumbing and laundry, but it 

can be a health concern as well because it can cause neurological effects at elevated concentrations.(51, 154) Manganese is a metallic element that is present in igneous, metamorphic, and 

sedimentary rocks. Though common in aquifer rocks and sediments, manganese occurs in 

groundwater only when concentrations of dissolved oxygen are low.

Nationwide, concentrations of manganese were greater than the human-health benchmark 

of 300 μg/L in about 7 percent of the wells sampled in the parts of aquifers used for drinking 

water (table 4–1). Redox conditions were clearly important—manganese concentrations were 

much greater in anoxic groundwater (concentrations of dissolved oxygen less than 0.5 mg/L) 

than in oxic groundwater, and no concentrations greater than the benchmark were measured in 

oxic groundwater (fig. 5–15). Concentrations generally were higher in the eastern United States 

than in the West (fig. 5–16), reflecting broad regional patterns in redox conditions (fig. 3–9). 

Anoxic conditions are more common in the glacial, semiconsolidated sand, sandstone, and 

carbonate-rock aquifers—largely, aquifers in the eastern United States—than in the unconsolidated sand and gravel and basaltic-rock aquifers of the western United States. However, redox 

conditions can be quite variable within most aquifers, and manganese concentrations greater 

than the human-health benchmark were measured across the Nation and in at least one well in 

more than half the aquifers included in this study (fig. 5–17).

Figure 5–15. Manganese concentrations were 

much higher in anoxic groundwater than in oxic 

groundwater.

68 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 5

Figure 5–16. Manganese concentrations generally were higher in the eastern United States than in 

the West. Oxygenated groundwater, which limits manganese solubility, is common in the unconfined, 

unconsolidated sand and gravel and basaltic-rock aquifers of the western United States.

Median manganese concentration per study, in micrograms per liter

Manganese

Parts of aquifers used for drinking water

EXPLANATION

Each symbol represents a study network of 20 to 30 wells, 

mostly domestic supply wells

<1 1 to 30 >30 to 300 >300

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 69
Figure 5–17. Concentrations of manganese greater than the human-health benchmark of 300 μg/L 

were measured in at least one well in more than half the aquifers included in this study and in aquifers 

all rock types.

Basin and Range carbonate

Rio Grande

High Plains

Basin and Range basin-fill

Columbia Plateau basin-fill

All aquifers

Early Mesozoic basin

Columbia Plateau basalt

Alluvial-Yellowstone

Central Valley

Mississippi River Valley Alluvial

Hawaiian volcanic

New England crystalline

Valley and Ridge crystalline

Rocky Mountain Front Range crystalline

Biscayne

Castle Hayne

Upper Floridan

Ozark Plateaus

Edwards-Trinity

Mississippian

Cambrian-Ordovician

Denver Basin sandstone

Lower Tertiary

Pennsylvanian

Piedmont and Blue Ridge crystalline

Coastal lowlands

Mississippi embayment

Northern Atlantic Coastal Plain

Southeastern Coastal Plain

Texas coastal uplands

Glacial

California Coastal Basin

Northern Rocky Mountain Intermontane

Surficial

Willamette Lowland

Alluvial-Upper Colorado

Snake River Plain basalt

Snake River Plain basin-fill

Piedmont and Blue Ridge carbonate

Valley and Ridge carbonate

Ordovician

Woodbine

Silurian-Devonian

Percentage of wells with concentrations

greater than the human-health benchmark

Median concentration by aquifer, 

in micrograms per liter

Principal Aquifer rock type

0 50 100 150 403020100 50

EXPLANATION

All aquifers

Basalt and other volcanic rocks

Crystalline

Sandstone and carbonate

Glacial

Unconsolidated sand and gravel

Semiconsolidated sand

Sandstone

Carbonate

Manganese in Principal Aquifers

80%

(off scale) 588

(off scale)

70 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 5

circ1360 Ch05 TDS_graphic1

Regional

Local

Spatial

scale of

influence

Groundwater

age

Climate

Geology

Dissolved Solids 

Is the water freshwater or salt water? This question is answered by the 

water’s concentration of dissolved solids, which is a basic characteristic of all 

natural waters. Freshwater generally has dissolved solids concentrations less than 

1,000 mg/L.(17) Even in freshwater, however, dissolved solids in water can cause 

problems that impair water use. These problems include unpleasant taste, higher 

water-treatment costs, accumulation of minerals in plumbing, staining, corrosion, 

reduced equipment lifespan, and restricted use for irrigation. Concentrations 

less than 500 mg/L are recommended by the USEPA for public water supplies to 

avoid these problems in drinking water. When used for irrigation, water with high 

dissolved solids can reduce crop yield because the dissolved salts make it more 

difficult for plants to extract water from the soil. Dissolved solids in irrigation water 

can cause salts to build up in soils and aquifers (salinization) and can eventually 

make the land unsuitable for agriculture (see chapter 7 for more information).

Climate, geology, and groundwater age influence concentrations of dissolved 

solids in groundwater. Climatic differences extend across the widest spatial scales 

and result in broad regional patterns in concentrations. Geology and groundwater 

age vary at spatial scales from regional (such as differences in rock type among 

regionally extensive Principal Aquifers) to local (such as differences in geology 

or groundwater age with depth in an aquifer). Human activities can also influence 

concentrations of dissolved solids at a variety of scales, across large irrigated areas or 

at the sites of individual septic systems or pumping wells. 

Natural waters can have a wide range of concentrations of 

dissolved solids.

Term Dissolved solids concentration, in 

milligrams per liter

Fresh Less than 1,000

Saline 1,000 to 35,000

Ocean water About 35,000

Brine Greater than 35,000

What are dissolved solids?

The sum of all the substances, organic and inorganic, 

dissolved in water is measured by the total concentration of 

dissolved solids in the water. Calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, nitrate, and silica typically 

make up most of the dissolved solids in water. Combinations 

of these ions—sodium and chloride, for example—form salts, 

and salinity is another term that commonly is used to describe 

the dissolved solids content of water. All natural waters contain 

some dissolved solids from contact with soils, rocks, and other 

natural materials.

Groundwater typically is considered freshwater, but saline 

groundwater is present in some aquifers near the coast, in arid 

climates, or at great depths. Slightly saline water (dissolved 

solids between about 1,000 and 3,000 mg/L) is used for domestic 

supply in areas where no other water source is available, but 

moderately saline water (dissolved solids between about 3,000 and 

10,000 mg/L) generally is too salty to drink.(133)

Saline groundwater lies beneath fresh groundwater across 

much of United States.(133, 134) In some cases, the deep saline 

groundwater comes from seawater that was buried with the 

sediments when they were deposited in ancient oceans and 

bays. In other cases, saline water is the result of interaction 

with deeply buried salt deposits or with other types of rocks and 

sediments over tens of thousands of years.(135–137) Deep saline 

groundwater can mix with the overlying fresh groundwater when 

it travels upwards naturally along geologic faults or at the ends 

of long, regional flow paths. Saline water also underlies fresh 

groundwater along the coasts in a mixing zone where groundwater meets ocean water. Pumping wells can draw saline water 

inland and upwards into freshwater aquifers; this problem, called 

saltwater intrusion, is common in coastal settings (see chapter 7 

for more information).

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 71
Climate affects dissolved 

solids because of 

differences in precipitation, 

evapotranspiration, and 

recharge rates; geology 

affects dissolved solids 

because some types of 

rocks are more resistant 

than others to weathering.

Climate affects concentrations of dissolved solids in groundwater through precipitation, 

evapotranspiration, and groundwater recharge. Where precipitation is low and evaporation rates 

are high, there is less water to dilute the products of rock weathering than in humid regions 

where precipitation and groundwater recharge are abundant. Consequently, dissolved solids 

are relatively high in the warm, dry parts of the western United States (fig. 5–18). Evaporation 

of shallow groundwater, where the water table is near the land surface, is another process that 

concentrates dissolved solids in groundwater in arid regions.

Geology affects dissolved solids concentrations because some types of rocks are more 

resistant to weathering than others. Some sedimentary rocks, such as shales, carbonate rocks, 

and evaporites, are more soluble and easily weathered than quartz-rich sandstones or crystalline 

rocks such as granites. Such easily weathered rocks in the mountains and basin-fill sediments 

of the Southwest contribute to the relatively high dissolved solids in unconsolidated sand and 

gravel aquifers in this region (figs. 5–18 and 5–19). In the East, concentrations of dissolved 

solids are relatively low in aquifers composed of materials that are resistant to weathering, 

such as the semiconsolidated sand aquifers of the Atlantic and Gulf of Mexico coastal plains 

and the crystalline-rock aquifers in the Northeast and mid-Atlantic region (fig. 5–19). Some 

of the highest concentrations of dissolved solids were measured in wells in the Lower Tertiary 

sandstone aquifer and the alluvial aquifer of the Yellowstone River Basin in Wyoming and 

Montana (fig. 5–19), which are in areas with abundant geothermal features. Geothermal water, 

another geologic source, typically is high in concentrations of dissolved solids and is found 

locally throughout the western States.(52)

Where the climate is very dry (top), salt deposits build up on the land surface from the 

evaporation of soil water and shallow groundwater. The groundwater left behind can have 

very high concentrations of dissolved solids as a result. Geothermal water—water that has 

been heated by magma or hot rocks in the subsurface (bottom)—contains high concentrations 

of dissolved solids, partly because most rock-water reactions are more intense at high 

temperatures. Mixing with geothermal water is another source of high dissolved solids in 

groundwater. Both photographs are from the Dixie Valley in central Nevada.

Photographs by Michael Rosen, USGS

72 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 5

Figure 5–18. Dissolved solids were generally higher in the southwestern and central United States and lower in the East and 

Northwest. Climate and aquifer rock type influence dissolved solids concentrations at national and regional scales.

Median dissolved solids concentration per study, in milligrams per liter

<250 250 to 500 >500 to 1,000 >1,000

Each symbol represents a study network of 20 to 30 wells, 

mostly domestic supply wells

< -63 to 0

> 0 to 12.5 

34 to 72

72 to 267

12.5 to 34

Modified from Healy and others(180)

Precipitation minus potential evapotranspiration, 

in inches per year

EXPLANATION

Basalt and other 

volcanic rocks

Crystalline

Sandstone and 

carbonate

Glacial

Unconsolidated sand 

and gravel

Semiconsolidated sand

Sandstone

Carbonate

EXPLANATION

EXPLANATION

Dissolved solids

Parts of aquifers used for drinking water

Climate

Principal Aquifer rock type

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 73
403020100 50

Median concentration by aquifer, 

in milligrams per liter

Principal Aquifer rock type

Basin and Range carbonate

Rio Grande

High Plains

Basin and Range basin-fill

Columbia Plateau basin-fill

All aquifers

Early Mesozoic basin

Columbia Plateau basalt

Alluvial-Yellowstone

Central Valley

Mississippi River Valley Alluvial

Hawaiian volcanic

New England crystalline

Valley and Ridge crystalline

Rocky Mountain Front Range crystalline

Biscayne

Castle Hayne

 Upper Floridan

Ozark Plateaus

Edwards-Trinity

Mississippian

Cambrian-Ordovician

Denver Basin sandstone

Lower Tertiary

Pennsylvanian

Piedmont and Blue Ridge crystalline

Coastal lowlands

Mississippi embayment

Northern Atlantic Coastal Plain

Southeastern Coastal Plain

Texas coastal uplands

Glacial

California Coastal Basin

Northern Rocky Mountain Intermontane

Surficial

Willamette Lowland

Alluvial-Upper Colorado

Snake River Plain basalt

Snake River Plain basin-fill

Piedmont and Blue Ridge carbonate

Valley and Ridge carbonate

Ordovician

Woodbine

Silurian-Devonian

0 200 400 600 800 1,000

EXPLANATION

All aquifers

Basalt and other volcanic rocks

Crystalline

Sandstone and carbonate

Glacial

Unconsolidated sand and gravel

Semiconsolidated sand

Sandstone

Carbonate

Dissolved solids in Principal Aquifers

Percentage of wells with concentrations

greater than the recommended value

1,302

(off scale) 92%

(off scale)

72% (off scale)

Figure 5–19. Median concentrations of dissolved solids were between 100 and 400 mg/L in most aquifers, 

but concentrations greater than 500 mg/L—the upper limit recommended by the U.S. Environmental 

Protection Agency for public water supplies—were measured in some wells in nearly all aquifers. 

Nationally, 18 percent of wells had concentrations greater than the recommended value. Dissolved solids 

at these concentrations can restrict the use of water for irrigation as well as for drinking.

74 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 5

Dissolved solids typically 

are higher in aquifers 

with long flow paths and 

old groundwater than 

in aquifers with short 

travel times.

Groundwater that has been in an aquifer for a long time has had more time to react 

with aquifer materials than has groundwater that has been recharged recently. Consequently, 

dissolved solids concentrations tend to be higher in aquifers with long flow paths and old 

groundwater than in shallow aquifers with relatively short travel times between recharge and 

discharge areas. For example, dissolved solids concentrations are high in deep, confined aquifers 

with long groundwater residence times in the glacial and Cambrian-Ordovician aquifer systems 

in the upper Midwest (figs. 5–18 and 5–19). Within an aquifer, concentrations of dissolved 

solids generally increase with depth in the aquifer and with distance along flow paths.

Human activities can add dissolved solids to recharging groundwater. Detergents, water 

softeners, fertilizers, road salt, urban runoff, and animal and human waste are some of the 

human sources that are delivered to groundwater by wastewater disposal, septic systems, 

or direct application to land surface. Irrigation can cause dissolved solids in groundwater to 

increase in arid and semiarid regions (see chapter 7). As a result, dissolved solids concentrations 

may be higher in shallow, recently recharged groundwater near the water table beneath urban, 

suburban, or agricultural areas than in shallow groundwater beneath undeveloped areas or in 

deeper groundwater.

0 5 10 15 20

Shallow groundwater 

beneath agricultural land

Shallow groundwater 

beneath urban land

Aquifers used for

drinking water

Chloride in the glacial 

aquifer system

Chloride concentration, in 

milligrams per liter

Chloride is a major component of dissolved solids, and is 

a good indicator of human influence on groundwater quality 

because it is present in wastewater and does not react with 

aquifer materials or other dissolved chemicals in dilute water. 

The application of road salt—sodium chloride, the same 

chemical as table salt—for deicing is also a major manmade 

source of chloride to groundwater in the northern United States. 

The use of road salt in the United States has more than doubled 

since 1980, while other uses of salt have remained stable or 

decreased.(138, 139)

In the glacial aquifer system, which extends across the 

northern United States, chloride concentrations were highest in 

shallow groundwater beneath urban areas, reflecting the use of 

deicing salt and the many other manmade sources of chloride 

in urban and suburban areas.(116) Shallow groundwater in the 

glacial aquifer system typically discharges to streams, providing 

the base flow that makes up most of the water in streams during 

periods of low flow. Elevated concentrations of chloride in 

groundwater discharging to streams are an ecological concern 

because chloride is toxic to aquatic organisms. Moreover, 

chloride concentrations in groundwater are increasing across 

the Nation (see chapter 8). Because of concerns about environmental effects and to reduce costs, many cities are taking 

measures to reduce salt applications or are introducing alternative deicing chemicals.(140)

Urbanization and chloride—A concern for streams 

Photograph by Steven Corsi, USGS

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 75
Dissolved Solids in the High Plains Aquifer—Climate 

Effects and Vertical Gradients 

The High Plains aquifer extends across a broad region of the central 

United States, from South Dakota to Texas. Concentrations of dissolved 

solids increase from northern parts of the aquifer to southern parts, 

reflecting differences in climate and recharge rates (fig. 5–20). Warm 

air temperatures in the south increase evaportranspiration and decrease 

recharge, leading to the accumulation of dissolved solids in the unsaturated 

zone and underlying groundwater.(53)

Concentrations of dissolved solids in the High Plains aquifer also 

vary with depth as a result of natural processes and human influences. 

Deep groundwater in the High Plains aquifer is thousands of years old, 

whereas groundwater near the water table is much younger.(53–56) Where the natural flow system 

in the High Plains aquifer is undisturbed and manmade sources are absent, concentrations of 

dissolved solids increase gradually with depth because the older, deep groundwater has had 

more time to react with aquifer sediments than the younger, shallow groundwater (fig. 5–21A). 

Parts of the High Plains aquifer are in contact with underlying geologic formations that contain 

deep saline groundwater.(57) In these parts of the aquifer, concentrations of dissolved solids can 

be very high. For example, concentrations near the bottom of the High Plains aquifer, at a site 

in the central High Plains, were more than 10 times greater than the concentrations of recently 

recharged groundwater at the water table (fig. 5–21B). The natural upwelling of saline water at 

this location is likely augmented by pumping deep wells for water supply, which draws water 

toward the wells from all directions. Conversely, where human activities, such as irrigated 

agriculture, are widespread, concentrations of dissolved solids at the top of the aquifer, near the 

water table, are likely to be higher than concentrations deeper in the aquifer (fig. 5–21C).

The High Plains aquifer is a sequence of layered sediments beneath the central United States. 

It is the Nation’s largest aquifer, providing 20 percent of all groundwater pumped nationally, 

and is the sole source of drinking water for most of the region’s population. This photograph 

shows an outcrop of the Ogallala Formation, which is the primary hydrogeologic unit of the 

High Plains aquifer.

Photograph by Bill Johnson, The University of Kansas, courtesy of the Kansas Geological Survey

76 Water Quality in Principal Aquifers of the United States, 1991–2010

High Plains 

aquifer
Chapter 5

0

20

40

60

80

100

100 200 300 400 500 100 300

500 3,600 3,800 4,000 100 200 300 400 500

Dissolved solids, in milligrams per liter

Depth in the aquifer, in percent of 

aquifer thickness

Water table Water table Water table

Bedrock Bedrock Bedrock

Aquifer Aquifer Aquifer

Vertical gradients in dissolved solids in the High Plains aquifer

A. North—Undisturbed site C. South—Agricultural site 

 with irrigation recharge

B. Central—Site with upwelling

 of deep, saline groundwater

Site locations

A

C

B

TX

OK

CO

NM

SD

WY

NE

KS

High Plains 

aquifer

Modified from McMahon and others(56)

250 to 500

> 500 to 1,000

< 250

> 2,000

> 1,000 to 2,000

Dissolved solids concentration in wells, in

milligrams per liter

EXPLANATION

EXPLANATION

Mean annual air 

temperature, 

in degrees 

Fahrenheit 

65 

40 

Climate and dissolved solids concentrations in the High Plains aquifer

IA

TX

CO KS

NM

NE

OK

WY

SD

Southern High

Plains

Central High

Plains

Northern High

Plains

Base from U.S. Geological Survey digital data, 1:100,000

Albers equal-area projection

Standard parallels N 29°30' and 

45°30', central, meridian W –96°

IA

TX

CO KS

NM

NE

WY

SD

Modified from McMahon and others(56)

Figure 5–20. Dissolved solids 

concentrations in the High 

Plains aquifer increase from 

north to south, following broad 

regional patterns in climate. 

Data are from domestic wells 

in the Ogalalla aquifer, the 

primary hydrogeologic unit in 

the aquifer system.

Figure 5–21. Dissolved solids concentrations typically increase with depth in the High Plains aquifer, but human activities can reverse 

the natural vertical gradient. A, Concentrations increase gradually with depth at a site in the northern High Plains in Lincoln, Nebraska, 

where the natural flow system is undisturbed and manmade sources are absent. B, Concentrations increase sharply near the bottom 

of the aquifer at a site near in the central High Plains in Cimarron, Kansas, where brackish water from underlying geologic layers has 

moved up into the aquifer because of regional groundwater flow and pumping of deep wells. C, In an agricultural area in the southern 

High Plains near Castro, Texas, irrigation recharge with higher concentrations of dissolved solids than would occur with natural 

recharge has increased concentrations in shallow groundwater, reversing the natural increase with depth.

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 77
Dissolved Solids in the Southwest Basin-Fill Aquifers—

Climate, Geology, Hydrology, and Human Activities All at 

Work

In the unconsolidated sand and gravel basin-fill aquifers of the southwestern United States, all the factors that can result in elevated dissolved 

concentrations are at work (chapter 7)—a dry climate, easily weathered 

rocks, long groundwater flow paths, and human sources that include 

irrigation, wastewater, and urban runoff.(28, 29) The hot, dry climate of the 

Southwest means that evapotranspiration rates are high; evapotranspiration 

concentrates the dissolved solids in precipitation, in artificial recharge, and 

in shallow groundwater in discharge areas. Sedimentary rocks that include 

carbonate and evaporite units are easily weathered and contribute to high 

concentrations of dissolved solids in parts of the basin-fill aquifers. Long 

flow paths mean that interactions between water and aquifer rocks or sediments can occur over 

long periods of time for all rock types. Irrigation and the infiltration of wastewater or urban 

runoff are intense in some heavily developed basins. 

Concentrations of dissolved solids can become so high in parts of the Southwest basin-fill 

aquifers that the groundwater is unsuitable for drinking, irrigation, or other uses.(29) Concentrations of dissolved solids greater than the recommended (SMCL) value for drinking water of 

500 mg/L were measured in 20 to 40 percent of wells sampled in the parts of the four basin-fill 

aquifers used for drinking water (Basin and Range basin-fill, California Coastal Basin, Central 

Valley, and Rio Grande aquifers or aquifer systems, figure 5–19). Concentrations of dissolved 

solids greater than the SMCL were measured in 40 to 60 percent of shallow wells beneath 

agricultural or urban land in the four Southwest basin-fill aquifers. Concentrations greater than 

10,000 mg/L are present in topographically low areas of closed basins, such as the Great Salt 

Lake Desert in western Utah and the Salton Sea in southern California (fig. 5–22).

78 Water Quality in Principal Aquifers of the United States, 1991–2010

circ1360 ch05_indexmap01

Basin and Range

basin-fill aquifers

California Coastal Basin aquifers

Central Valley

aquifer system

Rio Grande

aquifer system
Chapter 5

N

Dissolved solids in the Southwest basin-fill aquifers

Southwest

Principal Aquifers

NV

OR ID

CA

AZ NM

UT CO

 EXPLANATION

≤ 500

 501 to 1,000

 1,001 to 3,000

 3,001 to 10,000

 > 10,000

 

Concentration of dissolved solids 

in wells, in milligrams per liter 

Great Salt 

Lake

Salton Sea

Figure 5–22. High concentrations of dissolved solids are common in the Southwest basin-fill aquifers. Dissolved solids 

concentrations shown on this map were measured in water from more than 21,000 wells, primarily water-supply wells, sampled 

for numerous U.S. Geological Survey studies in the region. More than half of the wells had concentrations greater than the 

recommended upper limit of 500 mg/L.

Chapter 5: Understanding Where and Why Constituents From Geologic Sources Occur 79
Each symbol represents a study network of 20 to 30 wells, 

mostly domestic supply wells

EXPLANATION

Median hardness, in milligrams per liter as calcium carbonate

Hardness

Soft (< 60)

Moderately hard (60 to < 120)

Hard (120 to 180)

Very hard (>180)

Parts of aquifers used for drinking water

Hardness—A common problem for water supplies from groundwater sources 

Hardness refers to a property of water that makes it react poorly with soap—

forming scum rather than suds—and sometimes leave scale deposits in pipes and 

water heaters. Hardness is mostly caused by dissolved calcium and magnesium. 

Nationally, about 60 percent of groundwater samples from parts of aquifers used 

for drinking water had hard or very hard water, and about 20 percent had naturally 

soft water. Areas with hard groundwater usually also have high concentrations of 

dissolved solids because calcium and magnesium are two of the major components 

of dissolved solids in groundwater. Groundwater in carbonate-rock aquifers 

typically is hard because calcium and magnesium are released into groundwater 

when carbonate rocks dissolve. Hard water can be treated with water softeners, 

which replace the dissolved calcium and magnesium with sodium or potassium. 

Costs of household water softeners typically average from about $500 to $2,000 for 

installation, with additional costs for maintenance.(141)

Aquifers of several rock types in the western and central United States and carbonate and some 

sandstone aquifers in the East had hard or very hard water. These are many of the same aquifers and 

regions that have high concentrations of dissolved solids.

Photograph copyright istockphoto.com

80 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 6

Activities associated with agriculture, industry, and urbanization all can 

contribute contaminants to groundwater. Many of these contaminants, 

such as industrial solvents and pesticides, are manmade chemicals 

that have no natural sources. Others, such as nitrate and chloride, have 

geologic sources, but human activities greatly increase their concentrations in 

groundwater relative to natural levels. Where, when, and how chemicals are 

used influence the occurrence of these contaminants in groundwater. Chemical 

characteristics influence how contaminants are transported through soils and in 

groundwater; geochemical processes and biodegradation can alter contaminant 

concentrations as they move along flow paths. 

Chapter 6: Understanding Where and Why 

Contaminants From Human Activities Occur

This chapter describes 

the sources of and 

factors that affect 

nitrate, pesticides, and 

VOCs in the Principal 

Aquifers.
Photograph by Jeff Vanuga, USDA, NRCS

Modified from Puckett and others(181)

Fertilizer use and nitrate concentration 

in groundwater at recharge, 1940–2004

1930 1940 1950 1960 1970 1980 1990 2000 2010

0

10

20

30

40

0

10

20

30

40

50

60

Year

Nitrate concentration, in milligrams per liter as N

Fertilizer use

General trend 

of nitrate

concentrations

in groundwater

Fertilizer nitrogen use, in billions of pounds

Estimated nitrate concentration 

in groundwater at recharge 

Figure 6–1. Fertilizer use 

in the United States has 

increased greatly since the 

1940s, and nitrate concentrations in groundwater recharge 

in agricultural areas have 

generally followed this trend.

Nitrate

Synthetic fertilizer use, waste disposal, and fossil-fuel combustion have greatly increased 

the amount of biologically available nitrogen in the environment.(58, 59) As a result, concentrations of nitrate, the primary form of nitrogen in groundwater, have increased (fig. 6–1). Nitrate 

in groundwater used for drinking water is a health concern, and the USEPA has set an MCL 

for nitrate of 10 mg/L as nitrogen (N) to protect against methemoglobinemia, or blue baby 

syndrome.(199) Nitrate-rich groundwater can also cause problems when it is discharged into 

lakes, rivers, or the ocean because nitrogen stimulates algal and plant growth and can lead to 

anoxia and eutrophication. In most instances, concentrations of nitrate in groundwater that are 

high enough to pose human-health or ecological risks result from human activities. 

82 Water Quality in Principal Aquifers of the United States, 1991–2010

Nitrogen is an essential 

nutrient for all living 

things. It is abundant, as 

gaseous nitrogen (N2 ), in 

the air we breathe. Bacteria 

convert N2 to biologically 

available forms—organic 

nitrogen, ammonium, 

and nitrate. Most of the 

nitrogen in groundwater is 

in the form of nitrate. 
Chapter 6

Circ1360 ch06_ fig2 Nitrate box

Shallow groundwater

beneath agricultural land

Shallow groundwater

beneath urban land

Parts of aquifers used

for drinking water

Percentage of wells with 

nitrate concentration greater 

than the MCL

Nitrate concentration, in milligrams 

per liter as N

Threshold

indicating 

human influence

 MCL

0 5 10 15 20 0 10 20 30 40 50

Nitrate concentrations

Exceedances of the 

human-health 

benchmark

22%

4.2%

4.1%

See sidebar, Boxplots, p. 61

For more information on nitrate 

in the Nation’s groundwater, 

see Dubrovsky and others (25)

Nitrate concentrations were greater than the MCL of 10 mg/L as N in about 4 percent 

of wells sampled in drinking-water aquifers (fig. 6–2), placing nitrate among the top four 

contaminants in terms of how frequently concentrations exceeded human-health benchmarks. 

More than one-third of wells sampled in the parts of aquifers used for drinking water had 

nitrate concentrations greater than 1 mg/L as N, a level that indicates the likely influence of 

human activities in most parts of the Nation (see sidebar, Background nitrate concentrations, 

p. 84). Concentrations were considerably higher in shallow groundwater beneath agricultural 

land than in shallow groundwater beneath urban areas or in deeper groundwater used for 

drinking-water supplies: 22 percent of wells in agricultural settings had concentrations greater 

than the MCL, and nearly two-thirds had concentrations greater than 1 mg/L as N. Elevated 

concentrations of nitrate in shallow groundwater are a concern because shallow groundwater 

in some agricultural areas is used for domestic water supply. Further, shallow groundwater 

beneath agricultural or urban land can move downward into deeper parts of the aquifer that are 

used for drinking water or may contribute to excess nitrogen in streams and coastal waters.

Figure 6–2. Nitrate concentrations were higher in shallow groundwater beneath agricultural areas 

than in shallow groundwater beneath urban areas or in deeper groundwater used for drinking water.

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur 83
Physical

features

Nitrogen

inputs

Redox

conditions

Nitrate high in 

groundwater

Where and Why Are Nitrate Concentrations High?

Nitrogen use and release (nitrogen inputs) across the landscape, physical 

features that control how fast water flows through soils and aquifers, and redox 

conditions are the three main factors that influence nitrate concentrations in groundwater. Nitrogen inputs are widespread and include fertilizers applied to crops, 

lawns, and turf; septic systems; and precipitation and dry deposition. As a result, 

high concentrations of nitrate were distributed broadly across the Nation, especially 

in shallow groundwater beneath agricultural land (fig. 6–3). Nitrogen inputs from 

farm fertilizer are the largest source of nitrogen nationally.(61)

Physical features that enhance the permeability of sediment and rock, such 

as large amounts of sand and gravel, fractures, or karst features (see sidebar, Karst 

features and tile drains—Physical features that affect water infiltration can increase 

or decrease the susceptibility of aquifers to nitrate contamination, p. 87), allow water and any 

nitrate it contains to move relatively quickly from the land surface to the water table or from 

the water table to greater depths in the aquifer. High concentrations of nitrate in the Piedmont 

and Blue Ridge and the Valley and Ridge carbonate-rock aquifers (fig. 6–4), for example, 

reflect the rapid movement of nitrate from agricultural sources through karst features into the 

underlying aquifer. In contrast, soils that contain large amounts of clay and silt restrict the 

infiltration of water and nitrate to the water table. In the glacial aquifer system,(8) concentrations of nitrate are surprisingly low under agricultural lands in parts of Illinois, Indiana, and 

Ohio, despite large inputs of nitrogen fertilizer at the land surface (fig. 6–3), because of 

clay-rich soils and artificial drainage (fig. 6–2). Confining layers—low-permeability geologic 

layers—below the water table can protect underlying deep, confined aquifers from the downward movement of nitrate.

Background nitrate concentrations

What would be the concentrations of nitrate in groundwater 

without contributions from human activities? For many parts 

of the Nation, this is a difficult question to answer because 

the effects of human activities are pervasive and groundwater 

in undeveloped areas is not widely monitored. Nationally, 

groundwater nitrate concentrations in areas with minimal 

contributions from human activities are mostly less than 1 mg/L 

as N, based on samples from 419 wells in undeveloped 

areas.(25) Background nitrate concentrations in groundwater 

vary regionally, however, because of differences in geology, 

climate, precipitation chemistry, and biological processes. 

For example, nitrate concentrations as high as 3.2 mg/L as N 

have been measured in undeveloped areas of the High Plains 

aquifer,(54, 55, 142) and concentrations of 5 mg/L as N or higher 

have been measured in groundwater beneath nitrogen-fixing 

plants in the Sonoran Desert in the southwest.(29) In contrast, 

throughout the Upper Floridan and overlying surficial aquifers, 

nitrate concentrations are typically less than 0.2 mg/L as N or 

below detection in groundwater that has not been affected by 

human activities.(70, 143, 144)

84 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 6

Parts of aquifers used for drinking water

Shallow groundwater beneath agricultural land use

Nitrate

Median nitrate concentration per study, in 

milligrams per liter as N

Each symbol represents a study network of 

20 to 30 wells, mostly domestic supply wells

EXPLANATION

Shallow groundwater beneath urban land use

Nitrogen input

< 2,900

2,900 to 15,000

Nitrogen input from fertilizer, 

manure, and atmosphere—

Estimated for 1997, in 

pounds per square mile

> 15,000

EXPLANATION

Modified from Dubrovsky and others(25)

< 1 

1 to 5

> 5 to 10

> 10

Figure 6–3. High concentrations of nitrate were broadly distributed across the United States. Concentrations were especially high 

beneath agricultural land in the mid-Atlantic region, the Midwest, the Northwest, and in California. These areas have high nitrogen inputs 

and conditions favorable to nitrogen transport in groundwater. In shallow groundwater beneath urban land, concentrations of nitrate 

were elevated but with no clear regional patterns. Nitrate concentrations in the deeper parts of aquifers used for drinking water were 

high more frequently in the West than in the East (except for relatively high concentrations in the mid-Atlantic region, which reflect 

conditions in the shallow aquifers used for drinking water in this region).

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur 85
MCL

EXPLANATION

All aquifers

Basalt and other volcanic rocks

Crystalline

Sandstone and carbonate

Glacial

Unconsolidated sand and gravel

Semiconsolidated sand

Sandstone

Carbonate

Principal aquifer rock type

403020100 50

Basin and Range carbonate

Rio Grande

High Plains

Basin and Range basin-fill

Columbia Plateau basin-fill

All aquifers

Early Mesozoic basin

Columbia Plateau basalt

Alluvial-Yellowstone

Central Valley

Mississippi River Valley Alluvial

Hawaiian volcanic

New England crystalline

Valley and Ridge crystalline

Rocky Mountain Front Range crystalline

Biscayne

Castle Hayne

Upper Floridan

Ozark Plateaus

Edwards-Trinity

Mississippian

Cambrian-Ordovician

Denver Basin sandstone

Lower Tertiary

Pennsylvanian

Piedmont and Blue Ridge crystalline

Coastal lowlands

Mississippi embayment

Northern Atlantic Coastal Plain

Southeastern Coastal Plain

Texas coastal uplands

Glacial

California Coastal Basin

Northern Rocky Mountain Intermontane

Surficial

Willamette Lowland

Alluvial-Upper Colorado

Snake River Plain basalt

Snake River Plain basin-fill

Piedmont and Blue Ridge carbonate

Valley and Ridge carbonate

Ordovician

Woodbine

Silurian-Devonian

 

Aquifers 

used for 

drinking 

water Shallow 

groundwater 

beneath 

agricultural 

land use

Aquifers 

used for 

drinking 

water

Shallow 

groundwater 

beneath 

agricultural 

land use

60% (off scale)

0 2 4 6 8 10 12

Percentage of wells with 

concentrations greater than the MCL

Median concentration by 

aquifer, in milligrams per liter as N

Nitrate in Principal Aquifers

Figure 6–4. Nitrate concentrations were relatively high in several unconsolidated sand and gravel 

aquifers in the western United States and in agricultural areas of the Piedmont and Blue Ridge and 

the Valley and Ridge carbonate-rock aquifers in the mid-Atlantic region. These aquifers generally 

have high permeability, which promotes rapid infiltration of nitrogen-rich recharge, and generally have 

oxic groundwater, which favors the persistence of nitrate in the aquifer. In nearly all aquifers in which 

shallow groundwater in agricultural areas was sampled, that groundwater had higher concentrations of 

nitrate than did deeper groundwater used for drinking-water supply.

86 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 6

Karst

Some carbonate-rock aquifers have unique features called karst that make them very permeable. Carbonate rocks, mainly 

limestone and dolomite, are much more soluble than other rock types. As carbonate rocks dissolve, fractures and other open spaces in 

the rocks become enlarged, forming conduits, caverns, and sinkholes. Water from the land surface can enter aquifers directly through 

sinkholes and travel rapidly through conduits. These karst features make carbonate-rock aquifers highly susceptible to contamination 

from chemicals, such as nitrate, applied at the land surface.

Tile drains

Some aquifers lie beneath low-permeability, 

clay-rich soils that restrict water infiltration from 

the land surface. These soils are fertile, but their 

poor drainage properties make them unsuitable for 

agriculture. Low-permeability soils extend across 

much of the Corn Belt in the upper Midwest. When 

this land was developed for agriculture more than 

150 years ago, drainage was improved by installing 

drainage pipes below the surface, which are called 

tile drains because they were originally made out of 

clay. The tile drains collect water (and any chemicals 

it contains) from saturated soils and, where the 

water table is near the land surface, from shallow 

groundwater. The drains reroute the water to surface 

ditches and streams. Aquifers beneath such artificially 

drained soils are less susceptible to contamination by 

chemicals applied at the land surface than they would 

be without the drains. On the other hand, the rivers 

and streams receiving the tile drainage are more likely 

to contain higher concentrations of these chemicals 

than they would without the altered drainage.

cir1360_ch06no3_customsidebar2bottom

Tile drains remove excess soil 

water or shallow groundwater 

from poorly drained fields. 

Modern tile drains are perforated 

pipes several feet below land surface.

County with tile drains beneath

5 percent or more of the land area

Modified from Dubrovsky and others(25)

EXPLANATION

Tile drains

Photograph by Dana Kolpin, USGS

Photograph by Ann Tihansky, USGS

Water moves 

much more 

rapidly through 

conduits than 

through fractures.

Sinkholes are holes or 

depressions in the land 

surface that result from 

dissolving carbonate rocks. 

These enlarged openings 

allow water to bypass soil 

filtration and rapidly enter 

the aquifer.

Water table

Overland runoff

Sinkhole Sinkhole

Karst features

Rocks with karst features are widely 

distributed across most of the United States.

Modified from Epstein and others(145)

Wells that directly intersect a karst

conduit may draw in water that has

travelled quickly from the land

surface and may contain contaminants

from human activities.

Karst features and tile drains—Physical features that affect water infiltration can increase 

or decrease the susceptibility of aquifers to nitrate contamination

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur 87
Circ1360_ch06no3Fig05

See sidebar, Boxplots, p. 61

Anoxic

Oxic

EXPLANATION

Redox condition

0 5 10 15 20

Nitrate concentrations 

and redox conditions

MCL

Nitrate concentration, in milligrams 

per liter as N

Shallow groundwater

beneath agricultural land

Shallow groundwater

beneath urban land

Parts of aqufiers used for

drinking water

Nitrate can be removed 

in anoxic groundwater by 

denitrification, in which 

bacteria convert the nitrate 

to nitrogen gas.

Redox conditions influence nitrate concentrations because, although nitrate reacts little 

with aquifer materials, denitrification, a redox process, can removed nitrate in anoxic groundwater. Through denitrification, bacteria convert nitrate to N2

 gas. This process can happen only 

after the dissolved oxygen in the groundwater has been consumed and where organic carbon 

or other reduced chemicals are present to fuel the denitrification reaction (see sidebar, How 

do redox reactions work?, p. 33). Nitrate concentrations were lower in anoxic groundwater 

than in oxic groundwater in both shallow groundwater and in the deeper parts of aquifer used 

for drinking water (fig. 6–5), indicating that denitrification may play an important role in 

reducing nitrate concentrations in groundwater. Anoxic groundwater typically is older than oxic 

groundwater, however, and may be isolated from land surface by a confining layer; thus, anoxic 

groundwater may contain low concentrations of nitrate because the groundwater has not been 

affected by recent nitrogen inputs at land surface.

Detailed geochemical analyses can verify that denitrification has occurred in an aquifer. 

These analyses measure the gaseous products of denitrification in groundwater and the isotopic 

composition of the nitrate, which is changed by denitrification.(62, 63) In the glacial aquifer 

system in central Minnesota, for example, denitrification has substantially reduced nitrate 

concentrations that were originally 5 to 10 mg/L as N or even higher when the groundwater 

was recharged. Many parts of the glacial aquifer system are relatively enriched in organic 

carbon and are anoxic. Denitrification is more likely to occur in the glacial aquifer system than 

in other aquifers (fig. 6–6), such as the unconsolidated sand and gravel aquifers, where anoxic 

groundwater is much less common.(62, 63)

Figure 6–5. Nitrate concentrations were higher in oxic groundwater than in 

anoxic groundwater both in shallow groundwater underlying agricultural and 

urban land and in deeper groundwater used for drinking water. Anoxic conditions 

promote the removal of nitrate from groundwater through denitrification.

88 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 6

Photograph from the Morrisons Cove Herald, used with permission

Figure 6–6. At two sites in the glacial aquifer system in central Minnesota, denitrification has 

substantially decreased nitrate concentrations within decades of the groundwater entering the 

aquifer. Denitrification can occur at these sites because of anoxic conditions and the presence 

of organic carbon in the aquifer sediments.

The cost of nitrate contamination in 

rural Pennsylvania

Where groundwater used for public supply is contaminated 

with nitrate, treatment can decrease nitrate concentrations 

but is costly, particularly for small communities. An example is 

Martinsburg, a borough in rural Pennsylvania, where groundwater 

is the principal source of municipal water supply. The water supply 

for Martinsburg, which has a population of about 2,000 people, 

regularly has nitrate concentrations that exceed the MCL of 

10 mg/L as N (Randy Stoltz, Martinsburg Municipal Authority, 

written commun., 2012). Because there are no other water sources 

available, Martinsburg opened a water treatment facility in 2011 

to remove nitrate. The ion-exchange facility, the first of its type in 

Pennsylvania, cost $4.1 million, or more than $2,000 per person.

The nitrate removal plant in Martinsburg, Pennsylvania. 

Redox conditions, in 

percentage of wells

Oxic

Anoxic Mixed

Increasing

nitrate

removal

Data are from Böhlke and others(62) 

and Puckett and others(63)

EXPLANATION

Removal of nitrate by denitrification in the glacial aquifer system, central Minnesota 

Denitrification progress and 

groundwater age

Groundwater age, in years

0 20 40 60

Nitrate removed by 

denitrification, in percent 40

80

20

0

100

60

Nitrate concentration

 before denitrification,

 milligrams per liter as N

< 1

> 5 to 10

> 10 

> 1 to 5 

Study sites

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur 89
Modified from Nolan and Hitt(64)

< 1

1 to 5

> 5 to 10

> 10

Predicted nitrogen concentration, 

in milligrams per liter as N

Predicted nitrate in shallow, recently

recharged groundwater 

EXPLANATION

Predicted nitrate in deeper groundwater 

used for drinking water 

Figure 6–7. Nitrate concentrations greater than the MCL of 10 mg/L as N are predicted to occur primarily in the High Plains, northern 

Midwest, and areas of intense agriculture in the eastern and western United States. In shallow, recently recharged groundwater (left), 

concentrations are predicted to exceed the MCL of 10 mg/L as N beneath 2 percent of the land area of the conterminous United States 

and to be greater than half the MCL beneath nearly 8 percent of the land area. Concentrations are predicted to be lower in deeper 

groundwater from the parts of aquifers used for drinking water (right) than in shallow, recently recharged groundwater.

Measured nitrate concentrations, nitrogen sources, and factors associated with water 

transport and denitrification potential can be used to predict nitrate concentrations in groundwater across the United States(fig. 6–7).(64) These results can be used to identify areas that 

might warrant additional monitoring or are especially vulnerable to nitrate contamination. 

Nitrate concentrations in shallow, recently recharged groundwater are predicted to be greater 

than 5 mg/L as N (half the MCL) in groundwater beneath nearly 8 percent of the land area of 

the conterminous United States but predicted concentrations are lower in deeper groundwater, 

in the parts of aquifers used for drinking water (fig. 6–7). Nitrate concentrations also have been 

predicted at regional scales for the basin-fill aquifers of the southwestern United States,(31) the 

High Plains aquifer,(65) and the glacial aquifer system.(66)

90 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 6

Study

area

N

Piedmont and Blue Ridge 

carbonate

Valley and Ridge carbonate

Valley and Ridge siliciclastic

EXPLANATION

Principal Aquifer

Nitrate concentration, in 

milligrams per liter as N

< 1

1 to 5

> 10

> 5 to 10

MD

Modified from Lindsey and others(36)

VA

NJ

WV

Chesapeake

Bay

NY

Chesapeake Bay

watershed

PA

DE Figure 6–8. Carbonate-rock 

aquifers in agricultural areas 

of the mid-Atlantic region 

had some of the highest 

concentrations of nitrate in 

the Nation. These aquifers 

are heavily used for domestic 

water supply, and they 

contribute nitrate-enriched 

water to streams that drain to 

Chesapeake Bay.

Piedmont, Blue Ridge, and Valley and Ridge 

Carbonate-Rock Aquifers—Large Nitrogen Inputs, 

High Permeability, and Oxic Groundwater Result 

in Nitrate Concentrations at Levels of Concern for 

Human Health 

Some of the highest nitrate concentrations in the Nation 

were measured in shallow groundwater in agricultural areas of the 

Piedmont and Blue Ridge and the Valley and Ridge carbonate-rock 

aquifers. Concentrations were greater than the MCL in 60 percent 

of wells sampled in Piedmont and Blue Ridge carbonate-rock 

aquifers and in 25 percent of wells sampled in the Valley and Ridge 

carbonate-rock aquifers (figs. 6–4 and 6–8). Although they are small 

in area, these carbonate-rock aquifers are among the most heavily 

used for domestic water supply in the United States.(6) Thus, high 

nitrate concentrations in these aquifers are a concern to human health. In contrast, groundwater 

in the adjacent Valley and Ridge siliciclastic-rock aquifers (aquifers composed of sandstone, 

siltstone, and shale) had some of the lowest concentrations of nitrate measured nationally, and 

no nitrate concentrations exceeded the MCL (fig. 6–8). In the carbonate-rock and siliciclastic 

aquifers, recharge is abundant, groundwater flow paths are relatively short, and groundwater is 

typically less than 60 years old—common characteristics that make all three of these aquifers 

susceptible to contamination from recent human activities. Why are concentrations of nitrate in 

the carbonate-rock aquifers so much higher than those in the siliciclastic aquifers?

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur 91

Valley and Ridge

siliciclastic

aquifers

Piedmont and Blue Ridge

carbonate aquifers

Valley and Ridge

carbonate aquifers
Figure 6–9. High nitrate concentrations in the Piedmont and Blue Ridge and the Valley and Ridge 

carbonate-rock aquifers result from large inputs of agricultural nitrogen at the land surface, highly 

permeable karst features in the soil zone that allow rapid movement of nitrate to the aquifer, and oxic 

groundwater that limits denitrification in the aquifers.

High nitrate concentrations in the carbonate-rock aquifers result from three factors—high 

nitrogen inputs, karst geology (carbonate rocks), and oxic groundwater (fig. 6–9). The 

carbonate-rock aquifers underlie valleys and low-lying areas where land use typically is 

agricultural or urban, whereas land that overlies the siliciclastic aquifers is either forested or a 

mix of land uses. Agricultural and urban areas have higher nitrogen inputs than do forested or 

mixed-land-use areas. Nitrogen inputs are particularly high near the wells that were sampled 

in the Piedmont and Blue Ridge carbonate-rock aquifers, even compared with nitrogen inputs 

near wells sampled in agricultural areas elsewhere in the Nation (fig. 6–9). This difference is 

largely because of nitrogen inputs from manure from confined animal feeding operations.(61)

Permeable karst features in the soil zone overlying the carbonate-rock aquifers allow rapid 

movement of water and nitrate to the aquifers (see sidebar, Karst features and tile drains—

Physical features that affect water infiltration can increase or decrease the susceptibility of 

aquifers to nitrate contamination, p. 87). Oxic groundwater, which is especially widespread in 

the Valley and Ridge carbonate-rock aquifers, preserves nitrate by preventing denitrification. 

In contrast to the carbonate-rock aquifers, concentrations of nitrate in the Valley and Ridge 

siliciclastic-rock aquifer are low because nitrogen inputs are not as high, karst features are 

absent, and oxic conditions are much less common compared with the carbonate-rock aquifers 

(fig. 6–9).

 

EXPLANATION

Redox conditions, in 

percentage of wells

Oxic

Mixed

Anoxic

U.S. Environmental Protection Agency 

Maximum Contaminant Level

Median for all samples of shallow 

groundwater beneath agricultural 

areas nationally

Piedmont and Blue Ridge

carbonate

Valley and Ridge

carbonate

Valley and Ridge

siliciclastic

0 10 20 30 50 0 5 10 20 25

Total nitrogen input within 

500-meter radius of well, in 

thousands of pounds

Median nitrate concentration, in 

milligrams per liter as nitrogen

Principal Aquifer Nitrogen input Nitrate concentrations

15

Redox

Conditions

40

See sidebar, Boxplots, p. 61

92 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 6

Chesapeake Bay, the largest estuary in the 

United States, provides a wealth of benefits, 

including commercial fisheries, recreation 

opportunities, and wildlife habitat. However, 

excessive nutrients, sediment, and other contaminants during many decades have degraded the 

water quality of the bay and threaten this valuable 

ecosystem.(146, 147) Each year, large anoxic areas, 

called dead zones, develop in response to excessive nutrients that stimulate algal blooms, which kill 

or weaken fish and shellfish. Coordinated efforts 

of Federal and State agencies, local governments, 

and other organizations and institutions to reduce 

nutrients and restore water quality began in the 

1980s. Improvements have been slow for a variety 

of reasons, one of which involves groundwater. 

Groundwater provides a large part of the water 

Photographs from top left clockwise: Bridge and heron, Jane Thomas, IAN, UMCES; boat, Ben Longstaff, IAN, UMCES 

(ian.umces.edu/imagelibrary/), used with permission; soft-shell crabs in barrel, copyright Ken Rygh, istockphoto.com

Nitrogen from discharging groundwater contributes to eutrophication 

in the Chesapeake Bay and delays water-quality improvements

and nitrogen in streams in the Chesapeake Bay 

watershed. On average, about 50 percent of 

the water and nitrogen load in streams in the 

watershed comes from groundwater.(22, 25, 148) 

Transport of nutrients from the land surface 

through an aquifer to streams can take decades; 

nitrogen in groundwater discharging to streams 

today is the result of nitrogen inputs at land 

surface in years past, when the groundwater 

was recharged. Consequently, water-quality 

conditions in the bay respond slowly to reductions in nitrogen from nonpoint sources in the 

watershed that is delivered to bay waters through 

groundwater discharge.(148) The delay that 

groundwater introduces is essential to consider 

in planning for the restoration of water quality in 

Chesapeake Bay.(24)

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur 93
AL

GA

SC

MS

Nitrate and confined conditions 

in the Upper Floridan aquifer

Modified from Berndt and others(197)

Semiconfined

Unconfined 

Confined

0.1

Concentration, in milligrams per liter as N

0.01 1 10

Redox conditions, in percentage of wells

Oxic

Mixed

Anoxic

Nitrate concentrations Upper Floridan

aquifer

Redox 

conditions

EXPLANATION

See sidebar, Boxplots, p. 61

EXPLANATION

Confined conditions

Unconfined

Semiconfined

Confined

Not mapped

< 0.1

0.1 to 1.0 >10.0

>1.0 to 10.0

Nitrate concentration, in 

milligrams per liter

as N

Upper Floridan Carbonate-Rock Aquifer—Confined 

Aquifer Conditions and Anoxic Groundwater Result in Low 

Concentrations of Nitrate

The Floridan aquifer system is the fifth most heavily pumped aquifer 

in the United States, supplying water for irrigation, public supply, and 

domestic use.(5) Agriculture—field crops, vegetables, and citrus—covers 

about 20 percent of the land overlying the aquifer; urban and suburban areas 

also are extensive. Despite these sources of nitrogen, nitrate concentrations 

in many parts of the Upper Floridan aquifer are very low (fig. 6–10). The 

low-permeability confining layers of sand, clay, and sandy limestone that 

cover much of the Upper Floridan aquifer protect it from contamination 

from the land surface and from the overlying surficial aquifers. Additionally, 

anoxic conditions in the confined parts of the Upper Floridan aquifers favor 

denitrification (fig. 6–10).

Figure 6–10. The presence or absence of low-permeability confining units is a major hydrogeologic control on the distribution of 

nitrate in the Upper Floridan carbonate-rock aquifer. In unconfined areas, nitrate from the land surface can enter and move through the 

aquifer quickly, resulting in higher nitrate concentrations than in confined parts of the aquifer. Anoxic conditions in the confined and 

semiconfined parts of the aquifer make it less likely that nitrate will persist in these areas.

94 Water Quality in Principal Aquifers of the United States, 1991–2010

Upper Floridan 

aquifer

Surficial aquifers
Chapter 6

Crystal clear springs, such as Rainbow Spring in Marion County, Florida, are fed by groundwater from the Floridan aquifer system 

and are vulnerable to contamination from increasing concentrations of nitrate.

Photograph by Alan M. Cressler, USGS

The presence of a confining layer is a key difference between the Upper Floridan 

carbonate-rock aquifer and the Piedmont and Blue Ridge and the Valley and Ridge carbonaterock aquifers in the mid-Atlantic region. Groundwater in large parts of the confined Upper 

Floridan aquifer is more than 10,000 years old,(67, 68) whereas groundwater is far younger in the 

mid-Atlantic carbonate rock aquifers—only 10 to 20 years old in some parts of the Valley and 

Ridge carbonate-rock aquifer.(69) The age differences in groundwater between these two aquifer 

systems demonstrate the effectiveness of the confining layer in limiting water movement and 

nitrate contamination from the land surface into the Upper Floridan aquifer. 

Nitrate concentrations in the unconfined parts of the Upper Floridan aquifer, although 

much higher than concentrations in the semiconfined or confined parts of the aquifer 

(fig. 6–10), are low relative to concentrations in other aquifers nationally that also are affected 

by human activities. Yet concentrations in the unconfined parts of the Upper Floridan aquifer 

are about 10 times greater than background concentrations in the aquifer and, consequently, 

pose problems for the springs and streams into which the groundwater discharges. Florida’s 

freshwater springs, which are fed by water from the Floridan aquifer system, are a natural 

resource of enormous economic, recreational, and ecological value.(70) Increasing concentrations of nitrate during the past 40 years in many springs threaten the ecological integrity and 

water quality of these valuable water resources.(71)

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur 95
Mobility

Use

Persistence

Geochemical

conditions

Pesticide or VOC is

detected in groundwater

Pesticides and Volatile Organic Compounds

Pesticides and volatile organic compounds (VOCs) are pervasive in modern life. 

They are used in agriculture, industry, transportation, and many day-to-day activities 

around the home. Thousands of different chemicals have been manufactured for use 

in the United States. Many of these chemicals are toxic and can pose human-health or 

ecological concerns in drinking water or in the environment. Groundwater samples from 

the Principal Aquifers were analyzed for about 240 pesticides and VOCs, with a focus on 

those that are most heavily used.

Pesticides and VOCs can reach groundwater through infiltration in recharge from 

the land areas where they are applied, through accidental spills or leaks, or through waste 

disposal. Which chemicals reach groundwater depends to a large extent on their use, but 

it also depends on the properties of the chemicals. Chemical characteristics—such as 

water solubility, volatility, density, and sorption properties—determine the mobility of 

a chemical through soils and aquifers. Many pesticides and VOCs degrade in soils and 

groundwater into other chemicals; a chemical’s resistance to degradation (persistence) 

also will determine if it is detected in groundwater. Mobility and persistence, in turn, are 

affected by geochemical conditions in the aquifer. 

Volatile organic compounds

(VOCs) are a class of 

chemicals that can easily 

volatize (evaporate). They 

are everywhere in modern 

life, in gasoline, paints, glues, 

cleaners, and personal care 

products. VOCs also are used 

in the manufacturing of many 

different products, including 

automobiles, electronics, wood 

products, and plastics, in dry 

cleaning, in refrigeration, and 

for some types of pest control.

Photographs copyright istockphoto.com

96 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 6

Parts of aquifers used

for drinking water

Shallow groundwater

beneath urban land use

Shallow groundwater

beneath agricultural

land use

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100

Percentage of wells with at least one compound detected

Detected

concentration

Detected

concentration

> 0.1 μg/L

Any

> 0.2 μg/L

Any

1.4% 1.5%

4.9% 2.5%

Pesticides VOCs

Exceedances of humanhealth benchmarks 

in wells

Pesticides VOCs

0.9% 0.6%

Detection frequency

Exceedance, 

in percentage of wells

EXPLANATION EXPLANATION

Pesticides are used in 

agriculture, in homes and 

businesses, on lawns and 

gardens, along roads, in 

recreational areas, and on 

pets and livestock. There are 

hundreds of different pesticide 

chemicals in use in the United 

States.(190)

Figure 6–11. Pesticide and VOCs were detected frequently in groundwater but rarely at concentrations greater than human-health 

benchmarks. Detections and exceedances of human-health benchmarks were more frequent in shallow groundwater beneath agricultural 

and (or) urban land than in deeper groundwater from parts of aquifers used for drinking water. Although not currently used for drinking water in 

most areas, shallow groundwater eventually will move deeper into the aquifer and could affect the quality of future drinking-water supplies.

Photographs: left to right, Connie Loper, USGS; copyright istockphoto.com

How Often Were Pesticides and VOCs Detected and Are They Health Concerns?

Pesticides and VOCs were frequently detected in groundwater, and all Principal Aquifers are 

vulnerable to contamination by these chemicals (fig. 6–11; appendix 3). Pesticides were detected 

in 32 percent and VOCs were detected in 40 percent of wells sampled in the parts of aquifers used 

for drinking water. Concentrations of individual pesticides and VOCs were mostly low (less than 

0.2 μg/L), however, and human-health benchmarks were rarely exceeded (table 4–1). Detections and 

benchmark exceedances were more frequent in shallow groundwater beneath agricultural and (or) 

urban land than in deeper groundwater used for drinking water (figs. 6–11 and 6–12). The shallow 

groundwater that was sampled represents recently recharged groundwater present in a large part of the 

United States that will move deeper into aquifers, potentially affecting future drinking-water supplies.

Although they rarely exceeded existing human-health benchmarks as individual chemicals, 

pesticides and VOCs often occurred in groundwater as mixtures. Mixtures of pesticides or VOCs 

were present in about 40 percent of groundwater samples from parts of aquifers used for drinking 

water, in 68 percent of samples of shallow groundwater beneath agricultural land, and in 82 percent 

of samples of shallow groundwater beneath urban land. Mixtures are a potential concern because 

the human-health effects of some chemicals may be additive.(72–74) There are millions of possible 

chemical mixtures in drinking-water sources, and their interactions are not well understood.(75, 76)

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur 97
None > 0 to < 10 10 to < 25 > 25

Frequency of detection of a pesticide (> 0.1 μg/L) or volatile organic compound (> 0.2 μg/L),

in percentage of wells. Each symbol represents a study network of 20 to 30 wells

EXPLANATION

–

Shallow groundwater beneath agricultural land use

Shallow groundwater beneath urban land use

Parts of aquifers used for drinking water

Pesticides VOCs

Figure 6–12. Detections of pesticide and VOCs were widespread across the Nation. Detections of pesticides were most 

frequent and widespread in shallow groundwater beneath agricultural areas, and detections of VOCs were most frequent 

and widespread in shallow groundwater beneath urban areas.

98 Water Quality in Principal Aquifers of the United States, 1991–2010
Solvents (30)

Trihalomethanes (4)

Gasoline oxygenates (4)

Organic synthesis compounds (18)

Gasoline hydrocarbons (18)

Fumigants (8)

Refrigerants (3)

> 0.1 μg/L for pesticides

and > 0.2 μg/L for VOCs

Any

Detected concentration

EXPLANATION

Pesticides

VOCs

Shallow groundwater

beneath agricultural land

0 10 20 30 40

0 10 20 30 40 706050

Shallow groundwater

beneath urban land

0 10 20 30 40

0 10 20 30 40 706050

Parts of aquifers used for

drinking water

0 10 20 30 40

0 10 20 30 40 706050

Detection frequency by chemical use group

Pesticides Pesticides

VOCs VOCs

Herbicides (72)

Insecticides (59)

Fungicides and others (19)

Chemical use group

Number in parentheses following

the group name is the number of 

chemicals in the group

Percentage of wells with detection of compounds from the use group

Figure 6–13. Herbicides were the most frequently detected pesticides in groundwater, and trihalomethanes and solvents were the 

most frequently detected VOCs in groundwater. Many pesticides and VOCs have been widely used across the United States and are 

mobile and persistent in groundwater.

Chapter 6

Which Pesticides and VOCs Were Detected and Why?

Herbicides used in agriculture were the most frequently detected type of pesticide. 

Atrazine (and its degradates), simazine, prometon, and metolachlor were the herbicides most 

frequently detected (fig. 6–13; table 6–1). These findings generally reflect patterns of chemical 

use. Agricultural herbicides account for about 80 percent of the total amount of conventional 

pesticides used in the United States, and atrazine and metolachlor have been among the most 

heavily used agricultural herbicides for the past several decades.(77, 78) Atrazine, simazine, and 

metolachlor were detected more frequently in shallow groundwater beneath agricultural land 

than in other settings, whereas prometon, an herbicide used along roads and rights-of-way, was 

detected more frequently in shallow groundwater beneath urban land (appendix 3).

Trihalomethanes, a group of chemicals that includes byproducts of water chlorination, 

and solvents were the most frequently detected types of VOCs (fig. 6–13). Trihalomethanes 

and solvents, along with gasoline hydrocarbons, were detected in some wells in nearly every 

aquifer (appendix 3). Chemicals used in organic synthesis, gasoline oxygenates, fumigants, 

and refrigerants were detected less frequently overall and in fewer aquifers. As with pesticides, 

patterns of chemical use help explain some of the differences in distributions of chemical 

groups in groundwater. Solvents, trihalomethanes, and gasoline hydrocarbons have been used 

for many decades throughout the United States.(79) In contrast, use of fumigants and gasoline 

oxygenates (additives to enhance fuel octane) has been limited to small areas of the country. 

Gasoline hydrocarbons are used in larger amounts than any other kind of VOC but were less 

frequently detected than trihalomethanes and solvents. The lower frequency of detections, 

despite higher use, is because gasoline hydrocarbons also are among the least soluble of the 

VOCs, tend to sorb to soil and aquifer solids, and biodegrade under oxic conditions,(14) illustrating the importance of chemical characteristics as well as use patterns in the distribution of 

chemicals in groundwater.

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur 99
Pesticides and VOCs use different 

assessment levels

The presence of pesticides and VOCs in groundwater 

was evaluated in two ways—detections at any concentration 

(without regard to differences in analytical reporting levels for 

individual chemicals) and detections at concentrations greater 

than specified values called common assessment levels. 

Detection frequencies at any concentration provide the best 

representation of chemical occurrence in the environment, 

whereas detection frequencies at common assessment levels 

provide the best approach for comparisons among different 

chemicals. The common assessment levels used in this 

circular are 0.1 μg/L for pesticides and 0.2 μg/L for VOCs. The 

different assessment levels for pesticides and VOCs reflect the 

differences in the laboratory methods used to analyze these 

two groups of chemicals.

Table 6–1. Pesticides and VOCs that were detected most frequently in groundwater.

[Based on detections at any concentration in samples from parts of aquifers used for drinking water and in samples of shallow groundwater beneath agricultural and urban land]

Compound Description and general use

Pesticides

Atrazine, deethylatrazine, deisopropylatrazine, and 

hydroxyatrazine

Herbicide, commonly used to control weeds in corn, sorghum, and soybeans, 

and its degradation products 

Simazine Herbicide, commonly used to control weeds in corn, orchards, turf and lawns, 

along roads and rights-of-way, and in nurseries 

Prometon Herbicide, used to control weeds along fences, buildings, roads, railways, 

and rights-of-way 

Metolachlor Herbicide, commonly used to control weeds in corn and soybeans, in nurseries, 

on turf and lawns, and along fences 

VOCs

Chloroform Byproduct of the disinfection of water with chlorine and used in industry to 

produce other organic compounds; also has natural sources

Carbon disulfide Used in industry to produce other organic compounds and also used as an 

agricultural fumigant

Perchloroethene (PCE), Trichloroethene (TCE), and 

1,1,1-tricloroethane (TCA) 

Solvents with commercial and industrial uses, such as metal degreasing and 

dry cleaning

Methyl tert-butyl ether (MTBE) Gasoline additive (oxygenate), used to increase fuel octane and 

reduce emissions

100 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 6

circ1360_ch06_pv_cutsomsidebarfig1.ai

Parent herbicide concentration, 

in micrograms per liter

0.001 0.01 0.1 1 10

Degradates higher

0.01

0.1

1

10

0.001

1:1

Total degradate concentration, 

in micrograms per liter

EXPLANATION

Parents higher

Modified from Hopple and others(152)

Chloroacetanilides

Acetchlor

Alachlor

Metolachlor

Triazines

Atrazine

For more information on pesticides and VOCs in the Nation’s 

groundwater, see Gilliom and others(13) and Zogorski and others.(14)

C

C

C O

C

C

H

H H

H

H

H H

H H

H

H

H

Br

Br

Cl

C

C C

H

H

H

H

H

C Cl

Cl

Cl

H

Cl Cl

Cl Cl

C C

C

C

C C

H

H H

H H

H

C

H

H

C

C

H

H

C C H Cl

Cl

Cl

Perchloroethene

Toluene

Methyl tert -butyl ether

Chloroform

Dibromochloropropane

1,1,1-Trichloroethane

U.S. Department of the Interior

U.S. Geological Survey

Circular 1292

The Quality of Our Nation’s Waters

National Water-Quality Assessment Program

Volatile Organic Compounds in the Nation’s 

Ground Water and Drinking-Water Supply Wells

U.S. Department of the Interior

U.S. Geological Survey

Circular 1291

The Quality of Our Nation’s Waters

National Water-Quality Assessment Program

Pesticides in the Nation’s Streams and 

Ground Water, 1992–2001

Pesticide degradates

Many pesticides degrade in the soil or groundwater 

through physical, chemical, or biological reactions. This does 

not mean that the chemicals entirely disappear; rather, they are 

transformed into different chemicals. Some of these pesticide 

degradates can be as toxic as their parent chemicals and 

more mobile or persistent in groundwater.(149–151) Increasingly, 

laboratory methods are being developed that give us the ability 

to detect pesticide degradates in groundwater. The results show 

that pesticide degradates can be present just as frequently 

and sometimes at even higher concentrations than the parent 

chemicals. In a study of the source water to 73 public-supply 

wells, for example, concentrations of pesticide degradates 

equaled or exceeded those of the parent chemicals for two 

groups of herbicides: triazines and chloroacetanilides.(152) The 

triazine group includes atrazine, and the chloroacetanilides 

group includes several other commonly used agricultural herbicides. The chloroacetanilide herbicides break down more quickly 

in soil than the triazine herbicides.(13) As a result, concentrations 

of chloroacetanilide degradates almost always exceeded those 

of the parent chemicals, whereas concentrations of triazine 

degradates were about equal to those of the parent compound 

(atrazine). In some samples, the degradates were present even 

when the parent chemicals themselves were not detected.

Concentrations of herbicide degradates equaled or 

exceeded concentrations of the parent chemicals in 

many samples of source water to 73 public-supply 

wells in the Cambrian-Ordovician, glacial, High Plains, 

Piedmont and Blue Ridge crystalline, and Rio Grande 

aquifers or aquifer systems.

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur 101
Atrazine

Atrazine is frequently detected in groundwater for several reasons: it has been widely 

used for several decades, it is moderately soluble, and it has a relatively long half-life in soil 

(146 days).(13) Atrazine was detected in 40 percent of shallow wells sampled beneath agricultural land, in 29 percent of shallow wells sampled beneath urban land, and in 16 percent of 

deeper wells in drinking-water aquifers. Atrazine biodegrades, and its degradates, particularly 

deethylatrazine,(80, 81) were also among the most frequently detected pesticides (table 6–1). 

Few wells contained atrazine at concentrations greater than its USEPA MCL of 3 μg/L, but 

4.6 percent of shallow wells beneath agricultural land had atrazine concentrations greater than 

one-tenth of the MCL (0.3 μg/L).

Atrazine occurs so frequently that scientists can use its occurrence to learn about the 

factors that influence its distribution in groundwater. Two factors controlling distribution were 

the most important at the national scale: (1) where and how much atrazine was used, and (2) the 

age of the groundwater that was sampled.(82) Concentrations of atrazine and its degradate, 

deethylatrazine, were higher in groundwater beneath land areas of intensive atrazine use and 

in groundwater that had been in the aquifer for shorter periods of time than in groundwater 

beneath other land uses or in older groundwater. Groundwater age is important because atrazine 

Atrazine is an herbicide used in the production of corn, sorghum, and sugarcane; it also is used on 

lawns, turf, and tree farms. Atrazine has been one of the two most heavily used pesticides in the 

United States since at least 1987.(77, 78)

Photograph by Stephen Kirkpatrick, USDA, NRCS

102 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 6

circ1360_ch06_pv_fig04.ai

Predicted atrazine and deethylatrazine in

shallow groundwater beneath agricultural land

Atrazine use

Very low (< 0.09)

Low (0.09 to 4.5)

Medium (> 4.5 to 45)

High (> 45)

Estimated atrazine use, in pounds 

per square mile per year

< 0.001

0.001 to 0.01

0.01 to 0.1

> 0.1

Predicted atrazine and deethylatrazine concentration, 

in micrograms per liter

EXPLANATION EXPLANATION

Modified from Stackelberg and others(82)

use has increased with time, and older groundwater is less likely to have been recharged at a 

time when atrazine use was high than is younger groundwater. Several other factors also were 

related to concentrations of atrazine and deethylatrazine in groundwater. Higher concentrations 

of atrazine were measured in areas with more permeable soils and higher recharge rates than 

in areas with less permeable soils and lower recharge rates.(82) These factors reflect the ease 

with which water moves into the aquifer. Atrazine does not degrade readily in groundwater, and 

there was little evidence that atrazine was degraded once it reached the saturated zone.

Understanding the factors that control atrazine distribution in groundwater aids in 

predicting concentrations of atrazine and deethylatrazine in areas where no groundwater 

data are available (fig. 6–14). The highest concentrations are predicted to occur in shallow 

groundwater beneath agricultural land in parts of the High Plains aquifer, in unglaciated parts 

of Illinois, Iowa, Wisconsin, and Minnesota, and in southeastern Pennsylvania; concentrations 

are high in these areas because atrazine use is high, soils are permeable, and recharge rates are 

high.(82) Concentrations of atrazine and deethylatrazine are likely to exceed the atrazine MCL 

of 3 μg/L, however, in groundwater beneath only 5 percent of the Nation’s agricultural areas.

Predicted concentrations such as these can be used to identify areas of potential concern and set 

priorities for groundwater monitoring. 

Figure 6–14. Atrazine use (left) is one of the most important factors used to predict atrazine and deethylatrazine concentrations in 

groundwater (right). Within areas of high atrazine use, groundwater residence time, soil permeability, and other factors explain some of 

the differences in predicted concentrations.

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur 103
Table 6–2. Frequently detected VOCs.

Chemical

Detection frequency, 

at concentrations greater than 0.2 micrograms per liter,

as percentage of wells

Shallow groundwater 

beneath urban land

Shallow groundwater 

beneath agricultural 

land

Aquifers used for 

drinking water

Chloroform 20 2 6

Perchloroethene (PCE) 8 0.3 2

Trichloroethene (TCE) 4 0.2 1

1,1,1–Trichloroethane (TCA) 4 0.7 0.6

Chloroform, Perchloroethene, Trichloroethene, and Trichloroethane

Why were chloroform, perchloroethene (PCE), trichloroethene (TCE), and 1,1,1-trichloroethane (TCA) among the most frequently detected VOCs? These chemicals have numerous, 

widespread sources and a long history of use in the United States.(14) These VOCs also are 

moderately soluble in water, have a low tendency to sorb to aquifer materials, and are relatively 

slow to biodegrade, especially in oxic conditions. 

Chloroform is used in industry but also formed when water is treated with chlorine—when 

drinking water, wastewater, or pool water is disinfected, for example. Thus, leaky sewers, 

wastewater discharge, and landscape watering with chlorinated water are potential sources of 

chloroform to groundwater; these sources that are ubiquitous in residential and urban areas 

across the Nation. PCE, TCE, and TCA are chlorinated solvents with many commercial and 

industrial uses, including degreasing and dry cleaning. These VOCs enter groundwater through 

waste disposal, spills, and leaks. Chloroform, PCE, TCE, and TCA all were detected more 

frequently in shallow groundwater beneath urban land than in deeper groundwater used for 

drinking water or in shallow groundwater beneath agricultural land because sources of these 

VOCs are more common in urban areas than in other areas (table 6–2).

PCE and TCE were among the few pesticides or VOCs that were measured at concentrations greater than human-health benchmarks in groundwater samples. In fact, PCE and TCE 

accounted for nearly 20 percent of all instances in which a pesticide or VOC exceeded its 

human-health benchmark, second only to dieldrin in this regard (table 4–1). In contrast, 

chloroform was rarely present at concentrations of potential human-health concern; samples 

from only two wells had chloroform concentrations greater than the USEPA MCL (a combined 

MCL for chloroform and three other trihalomethanes).

104 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 6

Methyl Tert-Butyl Ether

Methyl tert-butyl ether (MTBE) is an oxygenate that was added to reformulated gasoline 

during the 1980s and 1990s to reduce air pollution. MTBE in water supplies quickly became 

a concern when it was detected in public wells in California and other States in the 1990s.(83)

Compared with other components of gasoline, MTBE is much more soluble, less likely to sorb 

to soils or aquifer materials, and more resistant to biodegradation. MTBE enters groundwater 

from leaking underground storage tanks and other releases of gasoline to the environment. 

There is no MCL or HBSL for MTBE, but the USEPA recommends concentrations in drinking 

water less than 20 to 40 μg/L to avoid unpleasant taste and odor.(60) A number of States have 

adopted MCLs for MTBE in drinking water that range from 10 to 70 μg/L.

Monitoring by the NAWQA Program documented the frequent occurrence of MTBE in 

groundwater as early as the mid-1990s, and these data helped inform USEPA recommendations 

to reduce the use of MTBE in gasoline.(84, 85) In the present study, which includes data collected 

from 1991 to 2010, MTBE was one of the most frequently detected VOCs in the Principal 

Aquifers. MTBE occurs mostly in shallow groundwater beneath urban areas, but only about 

1 percent of shallow wells beneath urban land had a concentration greater than 20 μg/L. The 

widespread occurrence of MTBE in groundwater (fig. 6–15), despite its relatively short history 

of intense use (fig. 6–16), illustrates how vulnerable shallow groundwater is to contamination 

by newly introduced chemicals with physical and chemical properties that make them mobile 

and persistent in the subsurface.

EXPLANATION

MTBE not detected or

detected < 2 μg/L 

MTBE detected > 2 μg/L

MTBE detections

MTBE in sampled wells

Figure 6–15. Methyl tert-butyl ether (MTBE) was widely detected across the United States. Nationally, MTBE was detected in 

18 percent of shallow wells beneath urban land, 5 percent of deeper wells in drinking-water aquifers, and 1.4 percent of shallow 

wells beneath agricultural land. It was detected most frequently, however, in wells in the Northeast, which was one of the areas 

of high MTBE use in the 1990s.

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur 105
Figure 6–16. Methyl tert-butyl 

ether (MTBE) production in the 

United States increased rapidly 

in the 1980s and 1990s because 

of its use in oxygenated gasoline. 

In 1998, MTBE ranked fourth 

in production among organic 

chemicals produced in the United 

States.(182) Production and use 

have greatly decreased since 

2000 partly because of concerns 

about contamination of drinkingwater supplies. The use of MTBE 

was restricted or banned in 

25 States as of 2007.(83)

MTBE in Santa Monica, California

MTBE contamination in Santa Monica, California, led to the closure in 1996 of supply wells that 

provided about 50 percent of the city’s water supply.(83) A new treatment plant was constructed 

to remove MTBE from the water supply and to restore the wells as a drinking-water source.(153)

Costs of more than $100 million dollars were incurred, including the costs of the treatment facility, 

of groundwater remediation and monitoring, and of about $3 million annually to purchase replacement water during the time the wells could not be used.(205, 206) MTBE production, in billion gallons per year

0

1

2

3

4

Year

MTBE production in the United States, 1982–2011

1980 1985 1990 1995 2000 2005 2010

NAWQA

groundwater

sampling period

Data from U.S. Energy Information Administration(183, 184)

Photograph courtesy of the City of Santa Monica

106 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 6

Fumigants

From a national perspective, agricultural fumigants are detected infrequently in groundwater. In most Principal Aquifers, they were not detected at all. However, in areas where they 

were used, fumigants such as dibromochloropropane (DBCP) and ethylene dibromide (EDB) 

are still detected in groundwater, and at concentrations higher than their human-health benchmarks, decades after their use was banned because of health concerns (fig. 6–17). 

Fumigants have been applied extensively in several areas of the United States since the 

1950s to control soil pests in agriculture. In those early days of pesticide use, groundwater was 

not thought to be vulnerable to contamination from chemicals applied at the land surface.(86)

This perception changed, however, when DBCP was detected in 1979 in California’s Central 

Valley aquifer system, 2 years after its use had been banned there because of reported sterility 

among manufacturing workers.(87, 88) Within a few years, DBCP was detected in groundwater 

in other States. The fumigants EDB, 1,2-dichloropropane (DCP), and 1,2,3-trichloropropane 

(TCP) also have long histories of use and were banned or unavailable for pesticide use by 

the early 1980s.(89–91) The fumigants DBCP, EDB, and DCP cause cancer and other health 

problems.(206)

In the Central Valley aquifer system in California, DBCP was detected in 11 percent of 

all wells sampled in the NAWQA Principal Aquifer assessments, and nearly three-fourths of 

the measured DBCP concentrations exceeded the USEPA MCL of 0.2 μg/L (fig. 6–17). In one 

groundwater study area in the Central Valley aquifer system, DBCP was detected in one-half of 

the domestic wells that were sampled, and about one-third of the sampled domestic wells had 

concentrations greater than the MCL.(29, 92) In the Hawaiian volcanic-rock aquifers underlying 

Oahu, DBCP and EDB were detected in more than 10 percent of wells, and DCP and TCP were 

detected in more than 30 percent of wells. Several of the fumigant concentrations measured 

in the Hawaiian volcanic-rock aquifers exceeded the Hawaii MCLs, which are lower than the 

USEPA MCLs.(93)

Figure 6–17. Nationally, agricultural fumigants were infrequently detected in groundwater, even at low concentrations. Where 

fumigant use was extensive, however, including parts of California (California Coastal Basin aquifers and Central Valley aquifer system), 

Washington (Columbia Plateau aquifers), and Hawaii (Hawaiian volcanic-rock aquifers), these chemicals were much more frequently 

detected in groundwater, even decades after their use was banned. (DBCP, dibromochloropropane; EDB, ethylene dibromide)

Chapter 6: Understanding Where and Why Contaminants From Human Activities Occur 107

Central Valley

Columbia Plateau

Hawaiian volcanic

All wells nationally

California Coastal Basin

0 10 20

Detection frequency, in percentage of wells

Small inset bar shows detections greater than 0.2 micrograms per liter

Fumigants in selected aquifers and nationally

0 10 20 0 10 20 30 40 50 0 10 20 30 40 50

DBCP EDB 1,2-Dichloropropane 1,2,3-Trichloropropane

Modified from Rupert and others(93)

California Coastal

Basin aquifers

Hawaiian

volcanicrock

aquifers

Central Valley

aquifer system

Columbia Plateau

basalt aquifers
Photographs clockwise from top left: USDA, ARS; U.S. 

Salinity Laboratory; copyright istockphoto.com

Fumigants are used to control worms, weeds, fungi, and other microorganisms in soil. They are 

injected as gas into the soil before planting (top left) or are mixed with irrigation water. Fumigants 

have been used extensively for a variety of crops, including pineapple on the Hawaiian island of 

Oahu (bottom), potatoes and sugar beets in the Columbia Plateau and Snake River Plain in the 

northwest, and orchards, vineyards, and other crops in California (top, right).

These agricultural fumigants are persistent in groundwater; samples for the NAWQA 

studies were collected between 1993 and 2002, about 25 years after the use of DBCP, EDB, 

DCP, and TCP as agricultural fumigants was discontinued. These VOCs remain in groundwater 

for a long time because they are highly soluble, do not sorb strongly to soils, and degrade very 

slowly. Forecasting models predict that concentrations of DBCP in Central Valley groundwater 

could continue to exceed the MCL for 70 years after the DBCP entered the aquifer(94) and that, 

similarly, DBCP could remain at detectable concentrations in Oahu groundwater for decades.(95)

In the Central Valley, DBCP and the other fumigants are likely to move deeper into the aquifer, 

potentially affecting public-supply wells in addition to the domestic wells in which they are 

currently detected.(96) Fumigants in source water to public-supply wells can require costly 

treatment when alternative water supplies are not available. Treatment systems for fumigants in 

a number of public-supply wells on Oahu, installed in the early 1980s at a cost of approximately 

$45 million, are still in operation, with ongoing annual maintenance costs of about $350,000.(97)

108 Water Quality in Principal Aquifers of the United States, 1991–2010
Photograph by Gene Alexander, USDA, NRCS

Chapter 7

About 80 billion gallons of groundwater is pumped each day from the 

Nation’s aquifers, and 128 billion gallons per day of water is spread 

across the landscape for irrigation. This movement of water has altered 

groundwater flow systems—profoundly in some cases, where the flux of water 

through aquifers has more than doubled relative to natural conditions prior to 

development. Whenever water is removed or added to an aquifer, groundwater 

flow directions, flow rates, and often geochemical conditions change. Consequently, when our use of water alters groundwater flow systems, groundwater 

quality also is affected, sometimes in unexpected ways. 

Chapter 7: How Does Our Use of Water Affect 

Groundwater Quality?

This chapter describes 

ways in which water 

use, particularly 

irrigation and groundwater pumping, have 

affected constituents 

in groundwater from 

geologic and manmade 

sources in a number of 

hydrogeologic settings.
Discharge 

Recharge

Change in

storage Modern

Predevelopment

Flow, in million gallons per day

0 5,000 10,000 15,000

Total flow

Modified from Bexfield and others,(28) Faunt,(155) and Campbell and Coes(156)

Modern

Predevelopment

Flow, in million gallons per day

0 5,000 10,000 15,000

Discharge 

Recharge

Change in

storage

Total flow

Predevelopment and modern water flows through aquifers

Central 

Valley

Northern

Atlantic and 

Southeastern

Coastal Plains Predevelopment Modern

Recharge

Discharge 

Central Valley aquifer system

Flow components

Northern Atlantic and Southeastern

Coastal Plain aquifer systems

Predevelopment Modern

Recharge

Discharge 

Flow components

Irrigation

Recharge sources

Flow from other aquifers

Precipitation

Leakage from streams

Flow to other aquifers

Evapotranspiration

Flow to streams

Discharge pathways

Pumping wells

EXPLANATION

Aquifer water budgets help us understand how much 

pumping, irrigation, and other activities have changed groundwater flow systems. A water budget quantifies the volumes of 

water that enter, leave, and are stored in an aquifer during a 

specified amount of time. Examples of annual water budgets are 

shown for a western aquifer system—the Central Valley aquifer 

system in California—and for an area along the east coast that 

includes part of the Northern Atlantic Coastal Plain and the 

Southeastern Coastal Plain aquifer systems.(28, 155, 156) Water use 

for agriculture, public supply, and other purposes associated 

with urban and residential development has substantially altered 

water flows in these aquifer areas.

In the Central Valley, total flows through the aquifer have 

increased sixfold. Groundwater recharge to the aquifer prior to 

development was from precipitation and leakage from streams. 

Discharge was mostly by evapotranspiration in wetlands and 

other low-lying areas. In modern times, irrigation accounts 

for nearly two-thirds of the annual recharge. Even with the 

additional recharge, the amount of groundwater pumped in the 

Central Valley is much larger than the annual recharge. This 

situation has led to a large net loss of the water stored naturally 

in the aquifer. Pumping also has left less water available to 

support wetland vegetation and wildlife. Groundwater levels 

have declined by as much as hundreds of feet, and aquifer 

compaction from the loss of water has led to subsidence of the 

land surface.(157)

In the Coastal Plain aquifer systems, total flows through 

the aquifer system have not changed greatly, but pumping has 

induced inflow of groundwater from other aquifers and reduced 

groundwater discharge to streams. Prior to development, 

recharge was entirely from precipitation and discharge was 

mostly to streams. In modern times, pumping accounts for about 

one-third of total annual discharges. Because groundwater 

discharge can contribute a large fraction of the flow in streams 

originating in the coastal plain, reductions in groundwater 

discharge are a potential concern for stream ecology.

How much have we changed groundwater flow systems?

110 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 7

Basin and 

Range basin-fill

California Coastal

Basin

Central Valley

Denver Basin

High Plains

Rio Grande

Eagle Valley

Truckee Meadows

Salt Lake Valley

Carson Valley

Sierra Vista

Upper Santa Cruz

Las Vegas Valley

Spanish Springs Valley

West Salt River Valley

Santa Ana Inland A

San Jacinto

Santa Ana Coastal

Santa Ana Inland B

San Luis Valley

Middle Rio Grande

1:1 2:1 3:1 4:1 5:1 6:1 7:1 8:1

Ratio of modern to predevelopment flows

Increases in water flows in selected western 

aquifers and basins—Predevelopment to 

Principal Aquifer and basin modern times

Recharge to aquifer

Discharge from aquifer

EXPLANATION

Data are from Bexfield and others,(28) 

Paschke,(185) and Stanton and others(186)

Large-Scale Flow Alterations in 

the Western Aquifers

The development of water resources for agricultural, 

urban, and residential uses in the arid and semiarid West 

has greatly increased both recharge to and discharge from 

the aquifers in these areas. Modern rates of recharge and 

discharge are more than twice the natural, predevelopment 

rates in some aquifers and basins (fig. 7–1). Infiltration of 

excess irrigation water and canal leakage are the two major 

sources of artificial recharge. Other sources of artificial 

recharge include leakage from water distribution pipes, sewer 

lines, and storm drains; septic-system effluent; and engineered 

infiltration of wastewater and stormwater. The increased 

discharge results primarily from groundwater pumping. 

Irrigation and pumping can accelerate the downward movement of manmade contaminants and increase concentrations 

of dissolved solids in groundwater.

Figure 7–1. Our use of water has drastically changed how water moves in the West. The amount 

of water that moves through aquifers—both recharge and discharge—has doubled, tripled, or 

increased by even more in some basins. These large changes in groundwater flow can affect the 

occurrence of contaminants from both manmade and geologic sources.

Photograph by Jeff Vanuga, USDA, NRCS

When crops and lawns are irrigated, not all the water is used by 

the plants. Some of the excess irrigation water moves downward 

into the aquifer, augmenting natural groundwater recharge and 

potentially carrying contaminants from the land surface. The 

photograph shows an irrigated wheat field in Yuma, Arizona.

Chapter 7: How Does Our Use of Water Affect Groundwater Quality? 111

1

2

5

3

4

6

7

8

9

1. Basin and Range aquifers

2. California Coastal Basin aquifers

3. Central Valley aquifer system

4. Denver Basin aquifer system

5. High Plains aquifer

6. Rio Grande aquifer system

7. Columbia Plateau aquifers

8. Snake River Plain aquifers

9. Hawaiian volcanic-rock aquifers

Aquifers or aquifer systems
Aquifers With Artificial Recharge Are More Vulnerable to Contamination 

From Human Sources 

Artificial recharge from irrigation and other sources increases groundwater flow rates and 

in many cases adds water to an aquifer in places where there was little or no natural recharge 

prior to development. This new recharge water can make the aquifers more vulnerable to 

human sources of contamination because the new recharge water brings contaminants down 

from the land surface into the aquifer.

The basin-fill aquifer in the San Luis Valley, which is part of the Rio Grande aquifer 

system in southern Colorado, is an example of where some of these changes have taken place 

(fig. 7–2). The infiltration of excess irrigation water and leakage from irrigation canals has 

increased recharge to more than three times the predevelopment rate. The additional recharge 

occurs across the valley floor in areas of irrigated agriculture that received little natural recharge 

in predevelopment times. The increased recharge has made the shallow aquifer more vulnerable to contamination from the land surface by providing a pathway for their transport into 

groundwater. Concentrations of nitrate have increased from less than 3 mg/L as N throughout 

Figure 7–2. In the San Luis Valley basin-fill aquifer, in southern Colorado, irrigated agriculture and groundwater pumping have 

increased water flow through the aquifer more than threefold relative to predevelopment conditions. As a result, the shallow, 

unconfined aquifer, which is used for domestic supply, is more vulnerable to human sources of contamination.

Irrigation

35% 

Canal leakage

22% 

Modern

Volcanic rocks

Volcanic rocks

Basin-fill aquifer

Confining layer

Precipitation

5% 

Leakage

from

streams

9% 

Native

evapotranspiration

29%

Mountain-front

recharge

12%

Flow to

streams

6%

Subsurface

recharge

8%

Flow 

to other

aquifers

4%

Pumping by wells

46% 

Inflow

from storage

5%

Evapotranspiration

from

irrigated fields

10%

Net

flow to

springs

2%

Precipitation

13% 

Leakage

from streams

18% 

Native

evapotranspiration

72%

Mountain-front

recharge

41%

Flow to

streams

14%

Subsurface

recharge

28%

Flow 

to other

aquifers

12%

Flow to 

springs

1%

Basin-fill aquifer

Confining layer

Predevelopment 

San Luis Valley, Colorado

Modified from Bexfield and others(28)

Water table

Water table

Size of arrows represents approximate volume of flow

NOT TO SCALE

NOT TO SCALE

Predevelopment

total flow

356 million gallons 

per day

1,200 million gallons 

per day

Modern

total flow

112 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 7

Confining layer

Fine-grained

deposits

Confining layer

Fine-grained

deposits

Basin-fill

sediments

Basin-fill

sediments

Marine deposits

Marine deposits

Coast 

Ranges

Coast 

Ranges

Sierra 

Nevada

Sierra 

Nevada

Consolidated

rock

Consolidated

rock

Native

evapotranspiration

85% Discharge to

streams, springs, 

and seeps

15%

Discharge to

streams, 

springs, and

seeps

16%

Central Valley, California

Pumping by wells

64% 

Precipitation

75% 

Precipitation

11% 

Predevelopment

Modern

Leakage

from streams

25% 

Leakage

from streams

20% Evapotranspiration

20%

Modified from Thiros(99)

Predevelopment

total flow

About 1,780 million 

gallons per day

About 11,900 million gallons

 per day recharge and 

about 13,000 million gallons 

per day discharge

Modern

total flow

Irrigation and

other artificial

recharge

69% 

NOT TO SCALE

NOT TO SCALE

Size of arrows represents approximate volume of flow

Figure 7–3. Infiltration of excess irrigation water and pumping from the deep aquifer have reversed natural flow directions in the 

Central Valley, California, so that groundwater flows downward and laterally, where it used to flow upward. As a result, the deeper 

aquifer, used for public supply and irrigation, is more vulnerable to human sources of contamination.

the basin before agricultural development to more than 10 mg/L as N over broad areas as the 

result of leaching of fertilizer applied to crops.(98) The deeper, confined aquifer, from which 

public-supply wells pump water, remains relatively unaffected by these changes. However, 

domestic wells commonly draw water for household use from the shallow, unconfined aquifer, 

which is affected by the recent human activities on the land surface.

In the Central Valley, California, recharge has increased more than sixfold and discharge 

has increased more than sevenfold with water and land development. Before development, 

groundwater flowed upward in the valley center and discharged in wetlands and streams 

(fig. 7–3).(99) Infiltration of excess irrigation water and pumping from the deep aquifer have 

reversed the direction of flow so that groundwater now flows downward throughout the valley.

(100, 101) In some areas, groundwater that previously discharged to the San Joaquin River, in the 

southern Central Valley, now flows laterally beneath the river toward pumping wells on the 

western side of the valley (fig. 7–3). Downward flow through a confining layer also is enabled 

by wells that are screened in upper and lower aquifer layers.(29) As a result, both shallow and 

deeper parts of the aquifer in the center of the valley are more vulnerable to contamination by 

manmade chemicals from the land surface.(92)

Chapter 7: How Does Our Use of Water Affect Groundwater Quality? 113
Salinization—the buildup of salts in soils to levels that are harmful to plants—is a worldwide 

problem and affects about one-fourth of the irrigated land in the United States.(191) Effects include 

reduced crop yield and restrictions on the kinds of crops that can be grown. Cotton for example 

(top left) is one of the more salt-tolerant crops grown in California. White salt deposits, from severe 

salinization, coat fields in the western San Joaquin Valley in California (middle). Salt-encrusted soils 

from the Colorado River Basin (bottom right). 

Dissolved Solids Increase in Aquifers With Artificial Recharge

Concentrations of dissolved solids are naturally high in groundwater and soil water in 

the arid and semiarid West (see “Dissolved Solids” section in chapter 5). Irrigation and other 

sources of artificial recharge, however, have increased those concentrations across widespread 

areas in parts of the Central Valley, California Coastal Basin, Basin and Range basin-fill, Rio 

Grande, and High Plains aquifers or aquifer systems.(28, 56) High concentrations of dissolved 

solids can restrict the use of the groundwater for drinking water or irrigation (see chapter 4). In 

the Santa Ana Basin in California, intensive water management for dissolved solids is needed to 

maintain drinking-water supplies (see sidebar, Groundwater in the intensively managed Santa 

Ana Basin, southern California, p. 115). High dissolved solids in irrigation water can reduce 

crop yields and contribute to soil salinization.

Concentrations of dissolved solids increase in groundwater as the result of artificial 

recharge for several reasons. The dissolved salts in irrigation water are left behind when the 

water evaporates or is taken up by plants, and percolation of excess irrigation water to the 

water table carries these salts to the groundwater. Excess irrigation water can flush minerals 

that have accumulated from thousands of years of weathering or evaporative concentration in 

the unsaturated zone down to the water table. Irrigation or other artificial recharge can raise the 

water table close to the land surface, so that direct evapotranspiration of shallow groundwater 

can further concentrate dissolved solids.

Photograph by Gary Bañuelos, 

USDA, ARS

Photograph by Scot t Bauer, USDA, ARS

Ron Nichols, USDA, NRCS

114 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 7

San 

Francisco

Los Angeles

San Diego

Santa 

Ana

Basin

California

Pacific Ocean

N

Dissolved solids and engineered recharge in the Santa Ana Basin, California

Sant a

Ana

River

Pacific Ocean

Modified from Thiros and others(29)

Dissolved solids concentration,

in milligrams per liter

EXPLANATION

< 250

250 to < 500

500 to <1,000

1,000 to < 3,000

3,000 to 25,000

Engineered recharge facility

Wastewater-treatment plant

Approximate direction 

of groundwater flow

Groundwater in the intensively managed Santa Ana Basin, southern California

Groundwater is a critical resource in the Santa Ana Basin 

in southern California, supplying two-thirds of the water used 

by the 5 million people who live there—about 1 billion gallons 

each day.(158) In parts of the basin, however, concentrations of 

dissolved solids in groundwater are higher than the SMCL of 

500 mg/L. More than 100 years of irrigated agriculture, natural 

processes such as evaporative concentration, and wastewater 

discharge contribute to these high concentrations of dissolved 

solids. Now, groundwater is intensively managed—through 

artificial recharge, desalination, high-level wastewater treatment, 

and saltwater intrusion barrier wells—to ensure that there is 

enough good-quality groundwater to supply the water needs of 

the population.

Engineered recharge facilities in the upper parts of the 

basin replenish the basin-fill aquifers with stormwater runoff and 

treated municipal wastewater to offset groundwater withdrawals. 

The recharged stormwater and wastewater typically have higher 

dissolved solids concentrations than the natural recharge, which 

is from mountain streams. Dissolved solids concentrations in the 

groundwater in these parts of the basin also are increased by 

excess irrigation water, especially irrigation water imported from 

the Colorado River.

Water from the Santa Ana River also is used to replenish 

the groundwater system. Much of the river’s flow is captured in 

the lower part of the basin and made to infiltrate into the aquifer. 

The river is the source of almost all the recharge to the aquifer in 

the lower part of the basin, but the river’s flow here is a mixture 

of treated municipal wastewater, intermittent stormwater runoff 

from agricultural and urban land, and discharge of groundwater 

that has been affected by human activities.(158) Pumping wells, 

desalination plants (for groundwater), and advanced wastewater 

treatment are being used to manage sources of dissolved solids to 

the river and to the aquifer.

Since 2008, water from a state-of-the art advancedwastewater treatment facility, which reduces dissolved solids in 

the effluent to less than 100 mg/L, is being used to recharge the 

aquifer at facilities near the Santa Ana River. This facility is the 

largest water purification project of its kind in the world and cost 

$480 million to build.(159) A $143 million expansion at the facility is 

scheduled for completion in 2014.

Recharge basins infiltrate Santa Ana River water into 

the aquifer

Chapter 7: How Does Our Use of Water Affect Groundwater Quality? 115
Modified from Jurgens and others(101)

Irrigation

well

Publicsupply

welll

Stream

NOT TO SCALE

Bicarbonate

EXPLANATION

Urban 

area

Agricultural area

Water table

Recharge Discharge

Groundwater flow 

 direction

Uranium in the Central Valley aquifer system

Silt and clay

Alluvium

Alluvium

Uranium

Figure 7–4. Uranium is mobilized in groundwater by irrigation recharge that contains high 

concentrations of bicarbonate in the Central Valley aquifer system, Modesto, California.

Groundwater Mixing and Geochemical Changes—Trace Elements Are 

Mobilized in Various Hydrogeologic Settings

Pumping, irrigation, and other flow alterations can mix waters from different sources or 

from different depths within an aquifer. If the compositions of the mixed waters are different, 

dissolved constituents can react with one another and with aquifer materials to release naturally 

occurring trace elements into the groundwater, potentially affecting human or ecosystem health.

Uranium in the Central Valley and High Plains Aquifers

In the Central Valley aquifer system near Modesto, California, uranium is mobilized (fig. 7–4) because of geochemical changes that result from agricultural and urban 

irrigation.(102, 103) Irrigation increases plant growth and microbial activity, which produce carbon 

dioxide. The carbon dioxide reacts with mineral solids to form bicarbonate in the soil water 

and shallow groundwater. Uranium occurs naturally in the aquifer; it comes from the granite 

rocks of the Sierra Nevada Mountains to the east, which are the source of the aquifer sediments 

in the valley. The uranium is more soluble in groundwater that contains bicarbonate because 

the bicarbonate chemically binds with the uranium to keep it in solution (see “Radionuclides” 

116 Water Quality in Principal Aquifers of the United States, 1991–2010

Central Valley 

aquifer system

Modesto, CA
Chapter 7

Low pH, high O2

Aquifer High pH, low O2

Confining layer

Shale

NOT TO SCALE

Public-supply

Irrigation well

well,

not pumping

Confining layer

Uranium

Modified from Landon and others(104)

EXPLANATION

Water table

Recharge Discharge

Groundwater flow 

 direction

Uranium in the High Plains aquifer

Aquifer

Aquifer

Figure 7–5. Uranium is mobilized in groundwater by the mixing of groundwater from different depths 

through long-screened wells in the High Plains aquifer, York, Nebraska. O2, dissolved oxygen.

section in chapter 5). The combination of irrigation and groundwater pumping increases the 

movement of the shallow, uranium-rich groundwater deeper into the aquifer. As a result, 

concentrations of uranium greater than the MCL are present in groundwater across the eastern 

San Joaquin Valley, and uranium in groundwater threatens the long-term sustainability of the 

aquifer as a source of drinking water.(102)

In the High Plains aquifer in York, Nebraska, pumping mixes water from shallow and 

deep aquifers,(103, 104) changing redox conditions and mobilizing uranium in the deep aquifer 

(fig. 7–5). The High Plains aquifer in this area is a layered sequence of aquifers separated 

by confining layers. The aquifer is heavily pumped to provide water for irrigation, and the 

pumping wells commonly are screened in more than one aquifer layer. Hydraulic head in the 

upper aquifer is higher than in the lower aquifer, so groundwater would flow downward if the 

confining unit were not present. When wells are not pumping, water can flow from the shallow, 

unconfined aquifer down through the well and into the deep, confined aquifer. Groundwater 

in the shallow, unconfined aquifer and groundwater in the lower, confined aquifer are chemically distinct; water in the shallow aquifer is oxygenated and has slightly acidic to neutral pH, 

whereas water in the lower aquifer is nearly anoxic and has slightly alkaline to neutral pH. 

When these waters mix in the deep aquifer, uranium is released from aquifer sediments into the 

groundwater. The mixed water with elevated uranium is pumped from a public-supply well that 

is screened in the deep aquifer. 

Chapter 7: How Does Our Use of Water Affect Groundwater Quality? 117

High Plains aquifer

York, NE
Figure 7–6. Arsenic in the carbonate rocks of the Upper Floridan aquifer is released to groundwater 

when recently recharged oxic groundwater is drawn down into the Upper Floridan aquifer by a highvolume pumping well near Tampa, Florida.

Arsenic in the Upper Floridan and overlying surficial aquifers

Arsenic

 Modified from Ayotte and others(103) and Katz and others(105)

Wetland

Wetland

High O2

High O2

Low O2

Low O2

Surficial aquifer

NOT TO SCALE

Upper Floridan carbonate aquifer

Confining layer

Publicsupply

well Sinkhole

Sinkhole

EXPLANATION

Water table

Recharge Discharge

Groundwater flow 

 direction

Arsenic in the Upper Floridan Aquifer

The limestones that make up the Upper Floridan aquifer contain small crystals of pyrite, 

a mineral that can contain arsenic. Pyrite is stable in anoxic water, but dissolves when exposed 

to oxic water, releasing the arsenic to groundwater. Mixing of anoxic and oxic water can 

occur inadvertently during high-volume pumping and during aquifer storage and recovery. 

High-volume pumping of a deep public-supply well in the Upper Floridan aquifer near Tampa, 

Florida, causes oxic water from the overlying surficial aquifers to be pulled down into anoxic 

water of the Upper Floridan aquifer (fig. 7–6). When the well was not being pumped, concentrations of arsenic were about 3 μg/L, but when the well was being pumped, concentrations 

were as much as 19 μg/L, which is almost twice the MCL of 10 μg/L.(105) Water from this 

well is blended with water from other public-supply wells so that arsenic concentrations in the 

distribution system remain below the MCL.

118 Water Quality in Principal Aquifers of the United States, 1991–2010

Upper

Floridan

aquifer

Surficial

aquifers

Tampa, FL
Chapter 7

cir1360_ch7_fig07.ai 

Selenium in groundwater

Alluvial

Aquifer

Dawson

Denver

Arapahoe

LaramieFox Hills

MCL

Redox 

conditions 

Oxic

Anoxic

Mixed

Redox conditions, 

in percentage of wells

0 50 100 150 300 400

Concentration, in micrograms per liter

Colorado

aquatic-life

standard

Increasing

depth

EXPLANATION

Selenium in the Denver Basin Aquifer System and Across the 

Irrigated Western States

Application of oxygenated irrigation water has enhanced the mobility of selenium 

in the shallow Denver Basin aquifer system and in parts of the West where selenium 

occurs in rocks and sediments. Selenium is particularly problematic for livestock and 

wildlife because it bioaccumulates and can be toxic at elevated levels.(106) Since the 

1930s, selenium has been known to cause sickness in cattle and horses where it is 

present in soils and plants. The association of high selenium levels and irrigation was 

first recognized in the 1980s, when waterfowl deaths and deformities at the Kesterton 

National Wildlife Refuge in California were linked to selenium in irrigation drainage 

water from Central Valley agricultural lands.(107–109)

In the shallow aquifers of the Denver Basin aquifer system, oxygenated recharge 

mobilizes selenium from rocks and sediments by converting the reduced form selenite 

to the more soluble and biologically available oxidized form selenate. Selenium concentrations in groundwater are higher in the shallow sandstone layers and overlying alluvial 

aquifer, where redox condition are mostly oxic or mixed, than in the deeper sandstone 

layers (fig. 7–7). Irrigation in agricultural and urban areas 

has increased recharge of oxygenated groundwater to the 

aquifer, and pumping from the sandstone aquifers has 

increased downward flow of the oxygenated water.(7, 110)

Figure 7–7. Selenium concentrations in the Denver Basin are much higher in the alluvial and shallow sandstone aquifers than 

in the deeper sandstone aquifers. Selenium in the rocks and sediments that make up the shallow aquifers is released when it 

interacts with oxygenated recharge from precipitation and irrigation. As a result, concentrations of selenium exceed the MCL 

for selenium much more frequently in the shallow Denver Basin aquifer system than nationally.

Sign posted at Sweitzer Lake State Park 

in the lower Gunnison River Basin in 

western Colorado. 

Photograph by Paul Diaz, Jr., USGS

Chapter 7: How Does Our Use of Water Affect Groundwater Quality? 119

Denver Basin

aquifer system
Salt deposits along the banks of Toll Gate Creek in Aurora, Colorado, 

contain selenium from the underlying bedrock and aquifer sediments. 

When streamflow levels and the water table rise during storm events 

and periods of groundwater recharge, the salts dissolve and release 

selenium to the streamwater. As a result, concentrations of selenium 

in Toll Gate Creek have been consistently above the Colorado aquaticlife standard since the early 2000s.(7, 110) Photograph from USGS

Salt deposits

These changes have the potential to move the selenium down into the deeper aquifers that 

are used for drinking-water supply. The shallow aquifers provide base flow to nearby streams 

and, in some areas, deliver concentrations of selenium that exceed the Colorado State standard 

(4.6 μg/L) for the protection of aquatic life (fig. 7–7).

120 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 7

Radium in the Northern Atlantic Coastal Plain Aquifer System

In the Northern Atlantic Coastal Plain aquifer system in southern New Jersey, radium 

mobility is enhanced by the application of ammonia-based fertilizers and by recharge of septicsystem effluent (fig. 7–8).(46, 103) The ammonia in these sources is oxidized to nitrate (a process 

called nitrification; see chapter 3) by microbes in the unsaturated zone and in oxic, shallow 

groundwater. Nitrification generates acidity, further lowering the naturally low pH in the 

aquifer. At low pH values (less than 5), the quartz-rich sediments of the coastal plain aquifers 

have little capacity to sorb cations, such as radium. Consequently, radium that occurs naturally 

in aquifer sediments is released to groundwater. Groundwater with elevated radium concentrations is drawn deeper into the aquifer by the pumping of public-supply wells, which withdraw 

large volumes of water. Although the aquifer materials do not contain large amounts of radium 

or its radioactive parent elements, the change in the groundwater chemistry is sufficient to 

mobilize the radium that is present. As a result, concentrations of radium in 21 percent of the 

public-supply wells sampled in the southern New Jersey study area were above the MCL for 

combined radium (see chapter 5).(46)

Publicsupply

well

Sand

Silt

High

bicarbonate

Low pH

Agricultural 

area

Clay

Sand

NOT TO SCALE

Coarse 

 sand

Radium

Low pH

Low pH

Ammonia

fertilizer

Nitrification,

increased acidity

Modified from Ayotte and others(103)

Radium in the Northern Atlantic Coastal Plain aquifer system

EXPLANATION

Groundwater flow 

 direction

Recharge

Discharge

Water table

Figure 7–8. In the Northern Atlantic Coastal Plain aquifer system in southern New Jersey, fertilizer application and septic-system 

effluent lead to decreases in pH that enhance radium mobility in the groundwater.

Chapter 7: How Does Our Use of Water Affect Groundwater Quality? 121

Southern

New Jersey

Northern Atlantic 

Coastal Plain 

aquifersystem
Figure 7–9. In the glacial aquifer system in southwestern Ohio, public-supply wells along the Great 

Miami River induce the infiltration of river water into the aquifer. Pesticides in the river can be drawn 

into the aquifer and mix with groundwater that is pumped by the supply wells.

Groundwater Mixing Across Aquifer Boundaries—Induced Infiltration 

and Saltwater Intrusion

Streams and the ocean are natural boundaries for groundwater flow systems. When flow 

systems are altered by pumping, flow directions can change, allowing groundwater to mix 

with seawater or river water. Pumping also can draw deep, saline groundwater up into parts of 

aquifers used for water supply.

Movement of Pesticide Compounds From Rivers To Aquifers in the Glacial 

Aquifer System

Many public-supply wells in the glacial aquifer system are near rivers. Thick, permeable 

glacial deposits in river valleys make these locations favorable for groundwater pumping. 

Pumping can reverse the natural direction of flow toward rivers, causing water to move from 

the river into the aquifer, a process called induced infiltration. When this process happens, 

the aquifer becomes vulnerable to contamination from chemicals in the river. One area where 

induced infiltration occurs is in the glacial aquifer system along the Great Miami River in 

southwestern Ohio.(111) Agricultural chemicals, including herbicides such as atrazine, are present 

in the river, especially during rainstorms during the times of year when the chemicals are used. 

Pumping of high-capacity public-supply wells can draw the river water and contaminants 

through the riverbed into the groundwater (fig. 7–9); river water can reach the wells within 

weeks or even days. As a consequence, pesticide compounds are detected more frequently and 

at higher concentrations in public-supply wells near streams than in domestic wells or shallow 

groundwater beneath agricultural areas in this part of the glacial aquifer system.(8)

Herbicides

in river

Public-supply

well

Domestic

well Monitoring

well

Herbicides

in publicsupply

well

Induced

infiltration

Herbicides in the glacial aquifer system

Water table

NOT TO SCALE Modified from Rowe and others(111)

EXPLANATION

Water table

Groundwater flow 

 direction

122 Water Quality in Principal Aquifers of the United States, 1991–2010

Glacial 

aquifer 

system
Chapter 7

1970 1975 1980 1985 1990 1995 2000 2005 2010

40

60

80

100

120

Chloride concentration, in milligrams per liter

100

150

200

250

Year

1970 1975 1980 1985 1990 1995 2000 2005 2010

EXPLANATION

25 to 75

76 to 100

101 to 200

201 to 300

>300

Water-level decline, 1870

to 2007, in feet

None

Supply well

Area of saline water in the

Middle Claiborne aquifer

Supply well in Union County, Arkansas

Supply well in Winn Parish, Louisiana

IL

MI

TN

AK

LA

AL

MO KY

Union County

Ouachita

Parish

!!

Memphis

Winn

Parish

Chloride in supply wells Water-level decline

Modified from Clark and others(187) and Kingsbury and others(33)

Figure 7–10. Large amounts of groundwater pumping during the past century have lowered water levels and caused saline water to 

move into areas of freshwater used for water supply in the Middle Claiborne aquifer, which is part of the Mississippi embayment–Texas 

coastal uplands aquifer system. Since the late 1990s, management actions to reduce groundwater withdrawals have reversed the 

water-level declines in some parts of the aquifer.

Upward Movement of Saline Water in the Mississippi Embayment–Texas 

Coastal Uplands Aquifer System

Deep, saline groundwater has moved upwards and inland in parts of the Mississippi 

embayment–Texas coastal uplands aquifer system as the result of pumping-induced declines 

in water levels. The freshwater in this aquifer system is underlain by saline groundwater that 

originated as seawater when the sediments were deposited. Large amounts of groundwater 

pumping since the early 1900s for public supply and industry have lowered groundwater levels 

by as much as 300 ft in parts of the aquifer system (fig. 7–10).(33, 112) When hydraulic heads are 

lowered by pumping in the freshwater aquifer, the saltwater can move upward or inland to parts 

of the aquifer used for water supply. This process is called saltwater intrusion. In supply wells 

in southern Arkansas and northern Louisiana, saltwater intrusion has caused chloride levels 

to more than double during the past 40 years (fig. 7–10). Saltwater intrusion also has affected 

groundwater supplies along the Atlantic coast from Maine to Florida(113) and along parts of the 

Pacific coast (see sidebar, Groundwater in the intensively managed Santa Ana Basin, southern 

California, p. 115). Mississippi Embayment–

Texas coastal uplands

aquifer system

Chapter 7: How Does Our Use of Water Affect Groundwater Quality? 123
Rangeland in the Nebraska Sandhills overlies the northern part of the High Plains aquifer.

Photograph from USGS

124 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 8

Concentrations of chemical constituents in natural waters—even in 

groundwater—can vary because of year-to-year climatic differences and 

because natural processes are inherently variable. In contrast, consistent 

change over time in a particular direction is a trend, and trends in contaminant 

concentrations raise concerns about the sustainability of groundwater use in 

future decades. Studying trends in groundwater quality at national or even 

regional scales is a large undertaking because of the expense of collecting 

consistent long-term data over large areas and because changes in response to 

contaminant inputs might not be apparent for many years. Identifying trends 

in groundwater quality and investigating their causes, however, is essential 

to helping water managers prepare for the future. Once contaminants in 

groundwater reach levels that impair its use, it takes a long time for reductions 

in contaminant inputs to restore the groundwater to its original quality, if such 

restoration is possible. 

Chapter 8: How Is Groundwater 

Quality Changing?

This chapter describes 

trends in groundwater 

quality and their 

implications for future 

groundwater-quality 

conditions.
Decadal Trends in Groundwater Quality, Early 1990s to 2010

Upward trends in concentrations of dissolved solids, chloride, and nitrate are indications 

of human influence on groundwater quality. Concentrations of all three of these constituents are 

increasing in many parts of the United States.(2, 3) Two-thirds of groundwater study areas had 

upward trends for concentrations of dissolved solids, chloride, and (or) nitrate (fig. 8–1); these 

trends were based on repeated sampling of the wells in groundwater study areas at 10-year 

intervals (fig. 8–1). Most changes in concentrations were measured in young groundwater 

(groundwater that was recharged since the early 1950s) because young, shallow groundwater 

is more likely to be affected by recent activities at the land surface than is older, deeper 

groundwater.(114)

The largest changes in concentrations of dissolved solids and chloride occurred in urban 

areas of the Northeast and upper Midwest and, for dissolved solids only, in agricultural areas 

in the Southwest and Florida (fig. 8–1). Urban sources of dissolved solids and chloride include 

wastewater disposal, stormwater runoff, and road-salt application. Irrigation contributes to 

increasing concentrations of dissolved solids in a number of aquifers in the West and Southwest 

(chapter 7). Upward trends in dissolved solids are of particular concern in arid areas of the 

western United States, where water supplies are scarce and dissolved solids concentrations in 

groundwater are naturally high, because of the potential limitations that high dissolved solids 

concentrations may place on future uses of groundwater. Upward trends in chloride concentration are a concern where groundwater discharges to streams because of the potential effects of 

high chloride concentrations on aquatic ecosystems.(115–118)

The largest changes in concentrations of nitrate occurred in shallow groundwater beneath 

agricultural areas. Increases in nitrate concentrations were mostly in oxic groundwater, in 

which denitrification cannot effectively reduce nitrate.(114, 119–121) In oxic groundwater, nitrate is 

similar to dissolved solids and chloride, generally—there are many natural and human sources 

that add these constituents to groundwater, but there are few natural processes that remove 

them. Upward trends in nitrate are a concern because of the limitations that elevated nitrate 

concentrations may place on drinking-water uses and because of the effects of nitrogen loads 

contributed by groundwater discharge to coastal waters.

126 Water Quality in Principal Aquifers of the United States, 1991–2010

A trend is a consistent 

change over time in a 

particular direction, 

distinct from the short-term 

changes that can result 

from seasonal effects, yearto-year climatic differences, 

or the inherent variability 

of natural processes.
Chapter 8

Decadal trends, early 1990s to 2010

Dissolved

solids

Chloride

Nitrate

Increase

No significant change

Decrease

< 0.1

0.1 to 0.5

> 0.5

EXPLANATION

Change in nitrate concentration,

 median of changes in study network, 

 in milligrams per liter as N

Median concentration in second 

 sampling event is greater than 

 10 milligrams per liter as N

Modified from Lindsey and Rupert(3)

Each symbol represents a groundwater 

study network of 20–30 wells

Increase

No significant change

Decrease

< 10

10 to 50

> 50

EXPLANATION

Change in dissolved solids concentration,

 median of changes in study network, 

 in milligrams per liter

Median concentration in second 

 sampling event is greater than 

 500 milligrams per liter

Increase

No significant change

< 1

1 to 20

> 20

EXPLANATION

Change in chloride concentration,

 median of changes in study network, 

 in milligrams per liter

Decrease

Figure 8–1. Concentrations of dissolved solids, chloride, and (or) nitrate in groundwater increased between the early 1990s and 

2010 in two-thirds of the groundwater study areas across the Nation. The studies targeted shallow groundwater beneath urban and 

agricultural areas (about two-thirds of studies) and deeper groundwater from drinking-water aquifers (about one-third of studies). 

Trends were determined statistically on the basis of samples collected from the same wells in the study area at 10-year intervals.

Chapter 8: How Is Groundwater Quality Changing? 127
Although most changes in groundwater quality between the early 1990s and 2010 were 

in shallow, young groundwater, similar changes are likely to occur in deeper parts of some 

aquifers in the future as the shallow groundwater moves downward over time. The deep 

groundwater in permeable, unconfined aquifers is especially vulnerable to change, for example, 

in the Central Valley aquifer system in California and in the surficial deposits of the Northern 

Atlantic Coastal Plain aquifer system (fig. 8–2). 

Changes in concentrations of manmade chemicals, such as pesticides and VOCs, in 

groundwater are closely related to their use. Data on chemical use are needed to understand the 

causes of these kinds of changes (see sidebar, Pesticide detections in groundwater in response 

to changing pesticide use, this page). However, information on historical use of pesticides and 

other manmade chemicals is not often available, either for specific areas or at the national scale, 

making it difficult to determine why concentrations have changed. At the national scale, three 

frequently detected pesticides—atrazine, deethylatrazine (a breakdown product of atrazine), 

and prometon—decreased in concentration from the early 1990s to the early 2000s, although 

the frequencies at which the compounds were detected in groundwater did not change.(122) The 

decrease in atrazine and deethylatrazine concentrations could be related to product changes 

in the early 1990s that resulted in less atrazine applied per unit area, but there is not enough 

information to know for sure. 

cir1360_ch8_customsidebar1_fig.ai

Modified from Choquette and others(160)

Photograph copyright istockphoto.com

1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012

Bromacil

Norflurazon

Demethyl norflurazon

Bromacil

use 

banned

0

50

30

10

40

20

90

60

70

100

80

Year

EXPLANATION

Frequency of detection, in 

percentage of samples

Pesticide detections in groundwater in response to changing 

pesticide use

Detections of bromacil, a herbicide applied to citrus crops, decreased in the 1990s and 2000s in 

the unconfined surficial aquifer in central Florida. These changes followed restrictions on bromacil 

use that began in 1994. Growers began to replace bromacil with norflurazon, a newer herbicide, 

in citrus orchards. Within a few years, norflurazon and its degradate desmethyl norflurazon were 

detected in groundwater more frequently than was bromacil.(160, 201) These changes in groundwater 

would be difficult to interpret without knowledge of the changing patterns of pesticide use.

128 Water Quality in Principal Aquifers of the United States, 1991–2010
Chapter 8

25

50

75

100

125

150

175

200

225

250

275

325

300

0

Monitoring

wells

Domestic

wells

Publicsupply

wells

Water

table

Agricultural Urban

Depth below land surface, in feet

NOT TO SCALE

NOT TO SCALE

Central Valley aquifer system, eastern San Joaquin Valley, California

MCL

<5 5–10 >10 

Nitrate concentration, in milligrams per liter as N

EXPLANATION

MCL

Median nitrate concentrations, 1993–95 and 2001–02

Northern Atlantic Coastal Plain surficial aquifer system, Delmarva Peninsula, Maryland and Virginia

1988 2001–03 1988 2001–03

15

10

5

0

15

10

5

0

1993–95 2001–02 1993–95 2001–02 1990s 2001–02

no data

1988 2000

Water table

0

25

50

75

100

Monitoring

wells

Domestic

wells Publicsupply

wells

Agricultural Urban

Depth below land surface, in feet

Domestic wells Public-supply wells

Shallow monitoring 

wells

Domestic wells Public-supply wells

Shallow monitoring 

wells

Modified from Dubrovsky and others(25)

Confining unit

Concentration, in 

milligrams per liter as N

Concentration, 

in milligrams per liter as N

Median nitrate concentrations, 1988 and 2001–03

Figure 8–2. Nitrate concentrations increased more in groundwater from shallow monitoring wells than in groundwater from domestic 

or public-supply wells in the Central Valley and Northern Atlantic Coastal Plain surficial aquifer systems, but over time, changes in 

shallow groundwater are likely to become apparent in deeper groundwater along the flow path.

Chapter 8: How Is Groundwater Quality Changing? 129
Forecasting Future Changes in Groundwater Quality

How will groundwater quality change in the future if we change our use and management 

of contaminants? In most aquifers, the slow movement of groundwater results in a lag time 

between contaminant inputs at the land surface and effects on water quality in supply wells and 

in streams that receive groundwater discharge. This lag time can be years to centuries long. 

Forecasting models, based on detailed field studies, can show how aquifers respond to 

changing contaminant inputs. Thin, shallow aquifers and those with physical features that 

promote rapid infiltration and flow respond more quickly than do thick, deep aquifers or those 

that are less permeable. Response times of simulated public-supply wells to increasing nitrate 

concentrations in the High Plains aquifer or Central Valley aquifer system, which are thick 

sequences of unconsolidated sediments, were three or more times as long as the response 

times of simulated public-supply wells in the thin glacial aquifer system or the karstic Upper 

Floridan aquifer (fig. 8–3). Developing models to accurately forecast future conditions requires 

a thorough understanding of the processes that might affect contaminant concentrations in 

groundwater. For example, when denitrification, a process that reduces nitrate concentrations 

in the aquifer, is included, the maximum nitrate concentrations predicted by the models are 

reduced by about half or more for all four aquifers.

130 Water Quality in Principal Aquifers of the United States, 1991–2010

Because groundwater 

moves slowly, there 

is a lag time, years to 

centuries in length, 

between contaminant 

inputs at land surface and 

effects on water quality in 

supply wells.
Chapter 8

Modified from Burow and Green,(188) McMahon and others,(189) and Dubrovsky and others(25)

Upper Floridan aquifer, Tampa, Florida

 Nitrate in groundwater

 from public-supply well

0

5

10

15

20

Nitrate in recharge

 MCL

Nitrate concentration, in 

milligrams per liter as N

0 20 40 60 80 100

Time, in years

0

5

10

15

20

Glacial aquifer system, 

Woodbury, Connecticut

Nitrate in recharge

 MCL

Nitrate concentration, in 

milligrams per liter as N

0 20 40 60 80 100

Time, in years

Nitrate in groundwater

 from public-supply well

York, NE

Central Valley

aquifer system

High Plains

aquifer

Modesto, CA

Tampa, FL

Woodbury, CT

Upper Floridan 

aquifer

Glacial aquifer system

Nitrate in groundwater

 from public-supply well

0

5

10

15

20

Central Valley aquifer system,

Modesto, California

Nitrate in recharge

 MCL

Nitrate concentration, in 

milligrams per liter as N

0 20 40 60 80 100

Time, in years

Nitrate in groundwater

 from public-supply well

0

5

10

15

20

High Plains aquifer, York, Nebraska

Nitrate in recharge

 MCL

Nitrate concentration, in 

milligrams per liter as N

0 20 40 60 80 100

Time, in years

Simulated nitrate concentrations in groundwater in four aquifers or aquifer systems

Figure 8–3. Forecasting models show how different aquifers respond to the same nitrogen inputs in recharge. In a thin 

glacial aquifer in Connecticut and in the Upper Floridan aquifer near Tampa, Florida, it takes only about 10 to 20 years for 

nitrate concentrations to reach the MCL of 10 mg/L as N after nitrogen concentrations reach this level in recharge. In 

contrast, it would take more than 60 years for nitrate concentrations to reach the MCL in the thick unconsolidated sand and 

gravel aquifers of the Central Valley aquifer system and High Plains aquifer. The location of the supply well in the aquifer 

also affects the response times.

Chapter 8: How Is Groundwater Quality Changing? 131
Year

Nitrate concentration in groundwater, 

in milligrams per liter as N

Gradual 

reduction of 

nitrogen inputs

after 2000

Nitrogen inputs

eliminated in 2000

1960 2000 2040 2080

0

5

1

2

3

4

1980 2020 2060

Constant nitrogen

inputs from 2000

Concentrations

increase in

response to

nitrogen 

inputs,

1940−

2000

Three model scenarios of future nitrate concentrations in 

public-supply wells, Northern Atlantic Coastal Plain 

aquifer system, New Jersey

1940 2100

Modified from Kauffman and others(123)

Forecasting models allow us to evaluate how our future actions, such as reducing or 

even eliminating contaminant inputs, might affect groundwater quality. Forecasting models 

for several aquifers predict that, even if nitrogen inputs were stabilized or eliminated, decades 

would be required for nitrate concentrations to stop increasing or to decrease to background 

levels. For example, models of nitrate concentrations in public-supply wells in the Northern 

Atlantic Coastal Plain aquifer in southern New Jersey indicate that concentrations would 

continue to increase until 2060, even if nitrogen inputs were held constant at the same levels as 

in 2000 (fig. 8–4). Similar projections are made for nitrate concentrations in the streams overlying the Northern Atlantic Coastal Plain aquifer system that receive groundwater discharge(123)

and at sites in basaltic-rock and unconsolidated sand and gravel aquifers in the western United 

States.(124, 125)

Figure 8–4. Models simulating nitrate concentrations in public-supply wells in the 

Northern Atlantic Coastal Plain aquifer system in southern New Jersey indicate 

that concentrations would increase for decades (green line), even if nitrogen inputs 

to shallow groundwater were held constant at the same levels as in 2000 because 

of the lag time between changes in contaminant inputs and the response of deep 

groundwater. Similarly, model simulations indicate that nitrate concentrations would 

be greater than background levels for decades after either gradual (blue line) or 

complete (red line) reductions in nitrogen inputs.

132 Water Quality in Principal Aquifers of the United States, 1991–2010
Looking Forward 

Water-quality trends observed since the early 1990s, the distribution of contaminants in 

shallow and deep groundwater, and groundwater forecasting models suggest that concentrations of contaminants from human sources are likely to increase in many parts of the Nation’s 

Principal Aquifers. Some of these changes will be the result of past actions—contaminant 

inputs in past years and decades that are not yet reflected in the quality of groundwater pumped 

from supply wells or discharged to springs, streams, and estuaries. Other changes will result 

from the management decisions made and actions taken today and in the future. Information 

on where contaminants occur, what are the sources of contaminants, and how contaminants are 

transported through groundwater is critical to understanding the limitations that contaminants 

may place on future water availability. 

For more information about NAWQA Principal Aquifer studies

This report characterizes groundwater quality for many of the Nation’s Principal Aquifers. 

Links to this and other circulars on the quality of groundwater in the 

Nation’s Principal Aquifers are available at

http://water.usgs.gov/nawqa/pasumm/. 

More than 2,000 NAWQA Program reports are available at 

http://water.usgs.gov/nawqa/bib/.

Photograph by Steve Krull, copyright istockphoto.com
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144 Water Quality in Principal Aquifers of the United States, 1991–2010
Glossary

Glossary

A

alluvial aquifer An aquifer composed of 

unconsolidated material, such as sand and 

gravel, deposited by a river or other flowing 

water.

anoxic Water with no dissolved oxygen or a 

very low concentration (less than 0.5 milligram per liter) of dissolved oxygen.

aquifer A geologic formation, group 

of formations, or part of a formation that 

contains a sufficient amount of saturated 

permeable material (for example, soil, sand, 

gravel and (or) rock) to yield substantial 

quantities of water to wells and springs.

artificial recharge Replenishment of an 

aquifer through human effort, for example, 

spreading water, recharge wells, or ditches; 

recharge through human activities that occurs 

at a rate greater than that of naturally occurring activities; the water artificially recharging 

an aquifer.

B

background concentration A concentration 

of a substance in a particular environment that 

corresponds to minimal influence by human 

(anthropogenic) sources or activities.

base flow Groundwater seepage into a 

stream or river. The continual contribution 

of groundwater to streams and rivers is an 

important source of streamflow between rain 

events.

bedrock General term for consolidated 

(solid) rock that underlies soils or other 

unconsolidated material.

C

carbonate rock or bedrock Rocks, such as 

limestone or dolostone, that are composed 

primarily of minerals, such as calcite and 

dolomite, containing the carbonate ion 

(CO3

2–).

cation An ion or group of ions that has 

acquired a positive charge by loss of one or 

more electrons.

closed basin An enclosed area having no 

drainage outlet, from which water escapes 

only by evaporation, as in an arid region.

common assessment level A single concentration threshold used to establish an equal 

basis for comparing detection frequencies 

among multiple chemicals. Use of a common 

assessment level avoids biases in detection 

frequencies caused by one compound having 

a lower detection level than another. Also 

sometimes referred to as a “common detection 

level.”

conduit Pipe-like or channel-like openings 

in bedrock. Conduits control the direction of 

water flow and greatly increase the speed at 

which water travels through an aquifer.

confined aquifer (artesian aquifer) An 

aquifer in which the groundwater is bounded 

between layers of relatively impermeable 

material, such as clay or dense rock. When 

tapped by a well, water in a confined aquifer 

is forced up, sometimes above the land 

surface, by pressure within the aquifer.

confining layer Geologic material with little 

or no permeability or hydraulic conductivity. 

Water does not pass through this layer or the 

rate of movement is extremely slow.

confining unit A hydrogeologic unit of 

impermeable or distinctly less permeable 

material within an aquifer or bounding one or 

more aquifers.

constituent A chemical or biological 

substance in water, sediment, or biota that 

can be measured by an analytical (laboratory) 

method.

contaminant For the purposes of this report, 

any manmade compound at any concentration or any constituent with a geologic source 

measured at a concentration exceeding the 

designated human-health benchmark.

crystalline rocks Igneous or metamorphic 

rocks consisting wholly of crystals or fragments of crystals. Granite and schist are 

examples of crystalline rocks.

Glossary 145
D

degradate A compound formed by the 

transformation of a parent compound, typically an organic contaminant or another 

degradate, by chemical, photochemical, or 

biological reactions. 

degradation The breakdown of a compound 

to a chemically simpler compound by abiotic 

or biotic processes.

denitrification The bacterial reduction of 

dissolved nitrate to nitrogen gas. Denitrification is the primary process by which nitrate 

can be eliminated naturally in groundwater.

desorption The release of a sorbed material 

from the solid to which it was sorbed. Opposite process of adsorption.

discharge The rate of flow of surface 

water or groundwater past a given point at a 

given moment, expressed as volume per unit 

of time. Also, the outflow from an aquifer, 

spring, or well or up through a streambed.

dissolution The process of dissolving a 

solid (mineral) into a homogeneous solution 

(water). Dissolution reactions result in the 

addition of ions to water as minerals react 

with water. Common dissolution reactions 

include dissolution of carbonate rock (limestone or dolomite) and incongruent dissolution of silicate minerals (feldspar) by carbonic 

acid (H2CO3). 

domestic well A privately owned well that 

typically serves one home and supplies water 

for human consumption and other homeowner 

uses.

drinking-water standard or guideline A 

threshold concentration in a public drinkingwater supply designed to protect human 

health or to identify acceptable concentrations 

of constituents that cause unpleasant taste, 

odor, or color in the water. 

E

eutrophication The enrichment of water by 

nutrients, most commonly phosphorus and 

nitrogen. During eutrophication, respiration 

processes that use organic matter cause a 

marked decline in dissolved-oxygen concentrations of water. 

evapotranspiration Loss of water from 

soil by evaporation and plant transpiration 

combined.

F

flow path The route or pathway of water 

flowing through the hydrologic system. Typically refers to subsurface (groundwater) flow.

fumigant A pesticide in the volatile organic 

compound (VOC) chemical class that is 

applied to soils to reduce populations of plant 

parasitic nematodes (harmful rootworms), 

weeds, fungal pathogens, and other soil-borne 

microorganisms.

G

gradient See hydraulic-head gradient.

groundwater Water that exists beneath the 

land surface, but most commonly refers to 

water in fully saturated soils and geologic 

formations. 

groundwater age The time elapsed since 

the recharge water became isolated from the 

atmosphere. The term “age” is normally qualified with the word “apparent” to signify that 

the accuracy of the determined age depends 

on many variables.

groundwater discharge The flow of water 

from the saturated zone, for example, from 

a spring or a well or as seepage to surface 

water.

groundwater flow path See flow path.

groundwater recharge The infiltration of 

water to the saturated zone. Also refers to 

water that reaches the water table by infiltration of precipitation or irrigation water 

through the unsaturated zone or by seepage 

of water from surface-water bodies, such as 

streams and lakes.

groundwater residence time The average 

amount of time it takes for groundwater 

to move from the point where it enters the 

aquifer to a specific point of discharge such as 

a well or stream. 

groundwater sustainability The amount of 

groundwater that will be available to support 

future uses of a particular aquifer or groundwater resource. The development and use of 

groundwater in a manner that can be maintained for an indefinite time without causing 

unacceptable environmental, economic, or 

social consequences.

H

half-life The time required for the concentration of a compound in a given environ146 Water Quality in Principal Aquifers of the United States, 1991–2010
Glossary

mental medium to be reduced to one-half of 

its original value by one or more processes, 

such as degradation or transport into another 

environmental medium.

Health-Based Screening Level (HBSL) An 

estimate of concentration (for a noncarcin-

ogen) or concentration range (for a carcin-

ogen) in water that (1) may be of potential 

human-health concern, (2) can be used as 

a threshold value against which measured 

concentrations of contaminants in ambient 

groundwater samples can be compared, and 

(3) is consistent with U.S. Environmental 

Protection Agency Office of Water method-

ologies.

herbicide A chemical pesticide designed to 

control or destroy plants, weeds, or grasses.

human-health benchmark A threshold 

concentration above which the concentration 

of a contaminant in drinking water could have 

adverse effects on human health. Treatment or 

other measures can be used before the water 

is consumed to lower the concentration of the 

contaminant below the benchmark.

hydraulic-head gradient In an aquifer, 

the rate of change of total head (water-level 

elevation in a well) per unit of distance of 

flow at a given point and in a given direction. 

Water will flow from higher hydraulic head to 

lower hydraulic head. 

hydrogeologic setting A unit with common 

hydrogeologic characteristics and therefore 

common susceptibility to contamination; 

a composite description of all the major 

geologic and hydrologic factors that affect 

and control the movement of groundwater 

into, through, and out of an area.

hydrogeologic unit A body of rock distin-

guished and characterized by its porosity and 

permeability. Also called a hydrostratigraphic 

unit.

hydrogeology The geologic and hydro-

logic features that control the movement of 

water, solutes, and small particles through the 

subsurface.

hydrologic system The assemblage of path-

ways by which water travels as it circulates 

beneath, at, and above the Earth’s surface 

during precipitation, runoff, evaporation, 

infiltration, transpiration, and groundwater 

discharge.

I

igneous rock Rock that solidified from 

molten (melted) or partly molten material. 

Granite is an example of an igneous rock.

induced infiltration Recharge to ground-

water by infiltration of water from the land 

surface as a result of the lowering of the 

groundwater head below the surface-water 

level, for example, by pumping.

infiltration Movement of water, typically 

downward, into soil or porous rock. 

intrinsic susceptibility A measure of the 

ease with which a contaminant in water enters 

and moves through an aquifer; a character-

istic of the aquifer and overlying material 

and hydrologic conditions independent of the 

chemical characteristics of the contaminant 

and its sources.

ion exchange The replacement of ions 

attached to the surface of a solid by ions 

that were in water. Ion-exchange reactions 

commonly occur between water and ions 

attached to clay particles. 

isotope Each of two or more forms of the 

same element with the same atomic number 

(the same number of protons) but a different 

number of neutrons and therefore different 

atomic weights. K

karst Surface and subsurface terrane that 

is formed on and in soluble rocks, such as 

limestone and gypsum, primarily by dissolu-

tion and collapse, and that is characterized by 

sinkholes, caves, and underground drainage.

karst aquifer A body of soluble rock 

that conducts water principally by way of 

a connected network of tributary conduits 

formed by the dissolution of the rock, which 

drains a groundwater basin and discharges to 

at least one perennial spring. L

land-use study A study by the U.S. 

Geological Survey National Water-Quality 

Assessment (NAWQA) Program to assess the 

effects of a specific land-use type (generally 

agricultural or urban) on groundwater quality, 

in most cases by sampling groundwater from 

monitoring wells that tap water from or near 

the water table.

Glossary 147
M

Maximum Contaminant Level 

(MCL) Maximum permissible level of a 

contaminant in water that is delivered to any 

user of a public water system. MCLs are 

enforceable standards established by the U.S. 

Environmental Protection Agency. 

median The middle or central value in a 

distribution of data ranked in order of magnitude such that one-half of the data are higher 

than the median and one-half are lower. The 

median is also called the 50th percentile. 

metamorphic rock Rock derived from 

mineralogical, chemical, or structural changes 

to preexisting rocks in response to marked 

changes in temperature, pressure, shearing 

stress, and chemical environment, generally 

at depth in the Earth’s crust. Gneiss and schist 

are examples of metamorphic rocks.

methemoglobinemia A health condition 

characterized by reduced ability of the blood 

to carry oxygen. Infants are most affected. 

One of the most common causes is nitrate 

in drinking water. Also called “blue baby 

syndrome.”

milligrams per liter (mg/L) A unit expressing 

the concentration of a chemical constituent 

as weight (milligrams) of constituent per unit 

volume (liter) of water; equivalent to one part 

per million in most streamwater and groundwater. One thousand micrograms per liter 

(μg/L) is equivalent to 1 mg/L.

monitoring well A well used to measure 

water quality or groundwater levels continuously or periodically. Not typically used as a 

source of drinking water. Sometimes referred 

to as an “observation well.”

N

natural attenuation The reduction in constituent concentration in the environment caused 

by naturally occurring physical, chemical, and 

(or) biological processes.

nitrate An ion consisting of one nitrogen 

atom and three oxygen atoms (NO3

–

). Nitrate 

is a plant nutrient and is very mobile in soils. 

nitrification The formation of nitrate by the 

oxidation of ammonium to nitrite followed by 

oxidation of nitrite to nitrate.

nonpoint source A contaminant source that 

is not a discrete point, such as a pipe, ditch, 

or tunnel. Areas of fertilizer and pesticide 

applications, atmospheric deposition, and 

stormwater runoff are examples of sources of 

nonpoint contamination.

nutrient An element or compound essential for animal and plant growth. Common 

nutrients include nitrogen, phosphorus, and 

potassium, such as are found in fertilizer.

O

organic carbon Carbon that originates from 

plants or animals and is bound in an organic 

compound.

organic matter Matter resulting from the 

decay of a plant or an animal and containing 

organic carbon compounds. Organic matter is 

rich in nutrients and is an essential component 

of soils.

organic compound A chemical compound 

containing the element carbon. A few types 

of carbon-containing compounds, including 

oxides of carbon (such as carbon dioxide, 

or CO2), are not considered to be organic. 

Organic compounds include petroleum products, solvents, and many pesticides.

oxic Water with a concentration of 

dissolved oxygen greater than or equal to 

0.5 milligram per liter.

oxidation The loss of electrons by a chemical species as a result of transfer to another 

chemical species, typically dissolved oxygen. 

The species donating electrons is “oxidized.”

P

permeability A measure of the relative ease 

with which a porous or fractured medium can 

transmit groundwater. Rock formations that 

transmit fluids readily are described as permeable.

permeable Capable of transmitting liquids 

or gases through pores or openings.

pesticide Any substance, organic or inorganic, used to kill plant or animal pests.

pH A measure of the acidity (pH less than 7) 

or alkalinity (pH greater than 7) of a solution; 

a pH of 7 is neutral. Formally defined as the 

logarithm of the reciprocal of the hydrogen 

ion concentration (activity) of a solution.

point source A stationary location or 

fixed facility from which contaminants are 

discharged, for example, a pipe, ditch, ship, 

ore pit, or factory smokestack.

precipitation Any or all forms of water 

particles that fall from the atmosphere such as 

148 Water Quality in Principal Aquifers of the United States, 1991–2010
Glossary

rain, snow, hail, and sleet. Also, the process in 

which a solid is formed from a fluid super-

saturated with dissolved ions. 

predevelopment The time prior to substan-

tial groundwater development by humans or 

effects of agricultural, urban, suburban or 

other human-related land uses. 

Principal Aquifer A regionally extensive 

aquifer or aquifer system that has the poten-

tial to be used as a source of potable water. A 

Principal Aquifer can be composed of one or 

more major aquifers.

public-supply well A privately or publicly 

owned well that provides water for public use 

to (1) a community water system, (2) a tran-

sient noncommunity water system, such as a 

campground, or (3) a nontransient, noncom-

munity system, such as a school. R

radioactive decay The spontaneous emis-

sion of particles (alpha or beta) and gamma 

rays from an atom with an unstable nucleus 

(radionuclide).

recently recharged groundwater Ground-

water that was recharged after 1952, as indi-

cated by tritium concentrations greater than 

0.5 tritium unit.

recharge The addition of water to the 

saturated zone, naturally, by precipitation or 

runoff, or artificially, by spreading or injec-

tion. Also, the water that is added.

reduction The gain of electrons by a 

chemical species, typically dissolved oxygen, 

as a result of transfer from another chemical 

species. The species accepting electrons is 

“reduced.” Once all of the dissolved oxygen 

has been reduced, other chemical species can 

accept electrons, following the most energeti-

cally favorable order. 

reduction-oxidation (redox) Chemical 

reactions that involve the transfer of elec-

trons from one chemical species to another, 

resulting in a change in the valence state of 

the species. Redox processes in groundwater 

often are microbially facilitated.

residence time The average amount of 

time that a solute, particle, organism, or other 

entity spends within a given environmental 

medium, such as a lake, groundwater, and the 

atmosphere.

runoff Excess rainwater or snowmelt that is 

transported to streams by flow over the land 

surface. S

saturated The condition in which all the 

pores (voids, interstices) within a material are 

filled with a liquid, typically water.

saturated zone The region in the subsurface 

in which all the spaces (pores and fractures) 

are filled with water and are under pressure 

greater than atmospheric pressure.

Secondary Maximum Contaminant Level 

(SMCL) Guidelines set by the U.S. Environ-

mental Protection Agency for concentrations 

of “nuisance” constituents in drinking water 

that may cause unwanted effects, such as 

unpleasant taste, color, or odor; discoloration 

of skin or teeth; or corrosion or staining of 

plumbing fixtures. Public drinking-water 

systems are recommended but not required to 

comply with these guidelines.

sedimentary rocks Rocks composed of 

particles derived from the erosion or weath-

ering of preexisting rocks or from chemical 

precipitation from water. Sandstone and lime-

stone are examples of sedimentary rocks.

siliciclastic rocks Rocks formed by the 

compaction and cementation of quartz-rich 

mineral grains. Sandstone and shale are 

examples of siliciclastic rocks.

sinkhole Any closed depression in soil or 

bedrock formed by the erosion and transport 

of earth material from below the land surface. 

A sinkhole typically has a closed topographic 

contour, drains to the subsurface, and occurs 

in karst terrane.

sorption The general process by which 

solutes, ions, and colloids become attached to 

solid matter.

subsurface The region of earth materials 

beneath the land surface that encompasses the 

soil and unsaturated and saturated zones.

susceptibility See intrinsic susceptibility. T

tile drain A drain installed to collect subsur-

face water and route it to a drainage ditch, 

stream, or wetland. Originally constructed 

using short segments of clay or cylindrical 

concrete “tiles” and installed manually, 

modern tile drains typically are corrugated, 

perforated plastic pipes installed 3 to 6 feet 

Glossary 149
below the soil surface by mechanical trenchers.

trace element An element found in only 

minor amounts (concentrations less than 

1.0 milligram per liter) in water or sediment; includes arsenic, cadmium, chromium, 

copper, lead, mercury, nickel, and zinc. 

tritium unit (TU) A measure of the concentration of tritium (3

H), equal to one 3H atom in 

1,018 atoms of hydrogen (H), or 3.24 picocuries per liter.

U

unconfined aquifer An aquifer that has a 

water table; an aquifer containing unconfined 

groundwater.

unconsolidated deposit Deposit of loosely 

bound sediment that typically fills topographically low areas. 

unsaturated zone A subsurface zone 

containing both water and air. The unsaturated 

zone is limited above by the land surface and 

below by the water table.

V

volatile organic compound (VOC) An 

organic chemical that has a high vapor pressure relative to its water solubility. VOCs 

include components of gasoline, fuel oils, 

lubricants, organic solvents, fumigants, some 

inert ingredients in pesticides, and some 

by-products of chlorine disinfection.

vulnerability The tendency or likelihood for 

contaminants to reach a specified position in 

the groundwater system after introduction at 

some location above the uppermost aquifer. 

The vulnerability of a groundwater resource 

to contamination depends both on the intrinsic 

susceptibility of the resource and on the 

locations and types of human and geologic 

sources of contaminants, locations of wells, 

and the characteristics of the contaminant(s).

W

watershed The upper surface of the saturated zone below which all voids (spaces) are 

filled with water.

water table The upper surface of the saturated zone below which all voids (spaces) are 

filled with water.

150 Water Quality in Principal Aquifers of the United States, 1991–2010
Appendixes

Appendixes 1–4

Appendix 1. Water-Quality Constituents Included in This Study

Appendix 2. Concentrations Exceeding Human-Health Benchmarks and 

Non-Health Guidelines in Principal Aquifers

Appendix 3. Detections of Pesticides and Volatile Organic Compounds in 

Principal Aquifers

Table A3–A. Pesticides detected at any concentration.

Table A3–B. Pesticides detected at concentrations greater than 0.1 microgram per liter.

Table A3–C. VOCs detected at any concentration.

Table A3–D. VOCs detected at concentrations greater than 0.2 microgram per liter.

Appendix 4. Groundwater Quality in Principal Aquifers in a National Context

—Concentrations by Study and Well Type

The appendixes for this report and the national dataset are available at http://pubs.usgs.gov/circ/1360/.

Appendixes 151
USGS Science Publishing Network 

Pembroke Publishing Service Center

Editors: Anna Glover and Kay Naugle

Technical illustration and graphic design: Christine T. Mendelsohn, 

Technical illustration: Susan Bergin and Mark Bonito

For more information concerning this report, contact:

Chief, National Water-Quality Assessment Program

U.S. Geological Survey

413 National Center

12201 Sunrise Valley Drive 

Reston, VA 20192

http://water.usgs.gov/nawqa/ 
ISSN 1067–084X (print)

ISSN 2330–5703 (online)

Printed on recycled paper http://dx.doi.org/10.3133/cir1360